diff --git a/docker/nginx-app.conf b/docker/nginx-app.conf
index 6c2f962..d44eccd 100644
--- a/docker/nginx-app.conf
+++ b/docker/nginx-app.conf
@@ -76,3 +76,10 @@ location = /healthz {
 location = /index.html {
     add_header Cache-Control "no-store, must-revalidate";
 }
+
+# The offline-access service worker (frontend/public/sw.js): a fixed URL the
+# browser re-checks for a new version, so it must not be cached either.
+location = /sw.js {
+    add_header Cache-Control "no-cache";
+    try_files $uri =404;
+}
diff --git a/frontend/public/sw.js b/frontend/public/sw.js
new file mode 100644
index 0000000..8fca783
--- /dev/null
+++ b/frontend/public/sw.js
@@ -0,0 +1,409 @@
+// Offline access (Settings → Security → "Keep recent mail on this device").
+//
+// Registered only on devices where the user turned it on. While the server
+// answers, everything comes from the network exactly as before; each answer is
+// also saved here. Only when the server cannot be reached is the saved copy
+// served instead — marked with `X-WM-Offline: 1`, which tells the page to show
+// that it is offline. Saved mail is the newest Inbox conversations (fetched by
+// the page, see lib/offlinePrefetch.ts) plus the ones opened recently, at most
+// MAX_THREADS; older ones are dropped.
+//
+// Saved mail is wiped when the server answers a request with 401 (signed out,
+// session expired or revoked), when another user signs in, and on request from
+// the page (sign-out, or the setting turned off).
+//
+// Plain JavaScript, served as is from /sw.js: a service worker must live at a
+// fixed URL, so it cannot be a hashed Vite chunk.
+
+const SHELL = "wm-shell-v1";
+const DATA = "wm-data-v1";
+const INDEX_KEY = "/__wm/offline-index";
+const APP_KEY = "/__wm/app";
+
+const MAX_THREADS = 80; // the newest 50 Inbox conversations + recently opened ones
+const MAX_ORPHANS = 50; // message parts whose conversation was never saved
+const MAX_LISTS = 40; // thread lists (one per folder / filter / page)
+const MAX_AVATARS = 300;
+
+// How long to wait for the server before falling back to the saved copy. Long
+// enough that a slow connection still gets fresh data; a missing one fails at
+// once anyway. Once a request has failed we know we are offline, and serve the
+// saved copy straight away until a request succeeds again.
+const API_TIMEOUT = 15000;
+const NAV_TIMEOUT = 8000;
+const RETRY_TIMEOUT = 4000;
+
+const META = new Set([
+  "/api/auth/me",
+  "/api/accounts",
+  "/api/mailboxes",
+  "/api/mailboxes/counts",
+  "/api/settings",
+  "/api/settings/quota",
+  "/api/saved-searches",
+  "/api/plugins/ui-manifest",
+]);
+const ID = "[0-9a-fA-F-]{36}";
+const THREAD_RE = new RegExp(`^/api/threads/(${ID})$`);
+const MESSAGE_RE = new RegExp(`^/api/messages/(${ID})/(?:html|attachments|cid/.+)$`);
+
+/** What a request is, for saving it; null = never saved (search, downloads, …). */
+function classify(url) {
+  const p = url.pathname;
+  if (META.has(p)) return { kind: "meta" };
+  if (p === "/api/threads") return { kind: "list" };
+  if (p === "/api/avatar") return { kind: "avatar" };
+  let m = p.match(THREAD_RE);
+  if (m) return { kind: "thread", id: m[1] };
+  m = p.match(MESSAGE_RE);
+  if (m) return { kind: "message", id: m[1] };
+  return null;
+}
+
+let offline = false;
+
+// ---- the index: what is saved, and when it was last fetched ------------------
+
+const emptyIndex = () => ({ user: null, threads: {}, msgs: {}, lists: {}, avatars: {} });
+let index = null;
+let chain = Promise.resolve();
+
+/** Run `fn` on the index, one at a time (fetches finish concurrently). */
+function withIndex(fn) {
+  const run = chain.then(async () => {
+    if (!index) {
+      try {
+        const hit = await (await caches.open(DATA)).match(INDEX_KEY);
+        index = hit ? await hit.json() : emptyIndex();
+      } catch {
+        index = emptyIndex();
+      }
+    }
+    const changed = await fn(index);
+    if (changed !== false) {
+      const cache = await caches.open(DATA);
+      await cache.put(
+        INDEX_KEY,
+        new Response(JSON.stringify(index), { headers: { "Content-Type": "application/json" } }),
+      );
+    }
+  });
+  chain = run.catch(() => {});
+  return run;
+}
+
+/** Forget everything saved (signed out, another user, setting turned off). */
+function purge() {
+  const run = chain.then(async () => {
+    index = emptyIndex();
+    await caches.delete(DATA);
+  });
+  chain = run.catch(() => {});
+  return run;
+}
+
+function oldestFirst(record) {
+  return Object.entries(record).sort((a, b) => a[1] - b[1]);
+}
+
+/** Record a saved response and drop whatever no longer fits. */
+async function record(ix, url, cls, body) {
+  const now = Date.now();
+  if (cls.kind === "thread") {
+    let ids = [];
+    try {
+      ids = JSON.parse(new TextDecoder().decode(body)).messages.map((m) => m.id);
+    } catch {
+      /* not the shape we know: keep the thread, without its messages */
+    }
+    ix.threads[cls.id] = { at: now, msgs: ids };
+    for (const id of ids) ix.msgs[id] = { t: cls.id, at: now };
+  } else if (cls.kind === "message") {
+    ix.msgs[cls.id] = { t: ix.msgs[cls.id]?.t ?? null, at: now };
+  } else if (cls.kind === "list") {
+    ix.lists[url.pathname + url.search] = now;
+  } else if (cls.kind === "avatar") {
+    ix.avatars[url.pathname + url.search] = now;
+  }
+  return trim(ix);
+}
+
+async function trim(ix) {
+  const dropThreads = new Set();
+  const dropMsgs = new Set();
+  const dropUrls = new Set();
+
+  const threads = Object.entries(ix.threads).sort((a, b) => b[1].at - a[1].at);
+  for (const [id] of threads.slice(MAX_THREADS)) dropThreads.add(id);
+  const orphans = [];
+  for (const [id, m] of Object.entries(ix.msgs)) {
+    if (m.t && (dropThreads.has(m.t) || !ix.threads[m.t])) dropMsgs.add(id);
+    else if (!m.t) orphans.push([id, m.at]);
+  }
+  orphans.sort((a, b) => b[1] - a[1]);
+  for (const [id] of orphans.slice(MAX_ORPHANS)) dropMsgs.add(id);
+  for (const [saved, max] of [
+    [ix.lists, MAX_LISTS],
+    [ix.avatars, MAX_AVATARS],
+  ]) {
+    const all = oldestFirst(saved);
+    for (const [u] of all.slice(0, Math.max(0, all.length - max))) {
+      dropUrls.add(u);
+      delete saved[u];
+    }
+  }
+  if (!dropThreads.size && !dropMsgs.size && !dropUrls.size) return;
+
+  for (const id of dropThreads) delete ix.threads[id];
+  for (const id of dropMsgs) delete ix.msgs[id];
+  const cache = await caches.open(DATA);
+  for (const req of await cache.keys()) {
+    const u = new URL(req.url);
+    const p = u.pathname;
+    const t = p.match(THREAD_RE);
+    const m = p.match(MESSAGE_RE);
+    if (
+      dropUrls.has(p + u.search) ||
+      (t && dropThreads.has(t[1])) ||
+      (m && dropMsgs.has(m[1]))
+    ) {
+      await cache.delete(req);
+    }
+  }
+}
+
+// ---- saving and serving ------------------------------------------------------
+
+/** Save a copy of a server answer (`res` is a clone the page never reads). */
+async function save(req, res, cls) {
+  const url = new URL(req.url);
+  const body = await res.arrayBuffer();
+  if (cls.kind === "meta" && url.pathname === "/api/auth/me") {
+    // Another user signed in on this browser: the saved mail is not theirs.
+    let id = null;
+    try {
+      id = JSON.parse(new TextDecoder().decode(body)).id ?? null;
+    } catch {
+      /* ignore */
+    }
+    await withIndex(async (ix) => {
+      if (ix.user === id) return false;
+      if (ix.user && id) {
+        Object.assign(ix, emptyIndex());
+        await caches.delete(DATA);
+      }
+      ix.user = id;
+    });
+  }
+  const headers = new Headers(res.headers);
+  headers.set("X-WM-Saved-At", String(Date.now()));
+  const cache = await caches.open(DATA);
+  await cache.put(url.href, new Response(body, { status: 200, headers }));
+  if (cls.kind !== "meta") await withIndex((ix) => record(ix, url, cls, body));
+}
+
+/** The saved copy for a request; conversations also match with other options. */
+async function lookup(req, cls) {
+  const cache = await caches.open(DATA);
+  const hit = await cache.match(req.url, { ignoreVary: true });
+  if (hit || (cls.kind !== "thread" && cls.kind !== "message")) return hit;
+  // The same conversation opened from another folder or in the other order, or
+  // a body asked for with/without remote images: still the same mail.
+  return cache.match(req.url, { ignoreVary: true, ignoreSearch: true });
+}
+
+function fromSaved(hit) {
+  const headers = new Headers(hit.headers);
+  headers.set("X-WM-Offline", "1");
+  return new Response(hit.body, { status: hit.status, statusText: hit.statusText, headers });
+}
+
+function notSaved() {
+  return new Response(
+    JSON.stringify({ error: { code: "offline", message: "Not available offline" } }),
+    { status: 503, headers: { "Content-Type": "application/json", "X-WM-Offline": "1" } },
+  );
+}
+
+async function network(req, ms) {
+  if (req.mode === "navigate") {
+    // A page load cannot be re-issued with a signal: just stop waiting for it.
+    let timer;
+    const late = new Promise((_, reject) => {
+      timer = setTimeout(() => reject(new Error("timeout")), ms);
+    });
+    try {
+      return await Promise.race([fetch(req), late]);
+    } finally {
+      clearTimeout(timer);
+    }
+  }
+  const ctrl = new AbortController();
+  const timer = setTimeout(() => ctrl.abort(), ms);
+  try {
+    return await fetch(req, { signal: ctrl.signal });
+  } finally {
+    clearTimeout(timer);
+  }
+}
+
+async function fromServer(event, req, cls, ms) {
+  const res = await network(req, ms);
+  offline = false;
+  if (res.status === 401) event.waitUntil(purge());
+  // Copied now, before the page starts reading the original.
+  else if (cls && res.status === 200) event.waitUntil(save(req, res.clone(), cls).catch(() => {}));
+  return res;
+}
+
+async function apiData(event, req, cls) {
+  if (offline) {
+    const hit = await lookup(req, cls);
+    if (hit) return fromSaved(hit);
+    // Not saved — and perhaps the connection is back: one quick try.
+    try {
+      return await fromServer(event, req, cls, RETRY_TIMEOUT);
+    } catch {
+      return notSaved();
+    }
+  }
+  try {
+    return await fromServer(event, req, cls, API_TIMEOUT);
+  } catch {
+    offline = true;
+    const hit = await lookup(req, cls);
+    return hit ? fromSaved(hit) : notSaved();
+  }
+}
+
+/** Every other API request: straight to the server, noting whether it answered. */
+async function apiOther(event, req) {
+  try {
+    const res = await fetch(req);
+    offline = false;
+    if (res.status === 401) event.waitUntil(purge());
+    return res;
+  } catch {
+    offline = true;
+    return notSaved();
+  }
+}
+
+// ---- the app itself ----------------------------------------------------------
+
+/** The files an app page loads up front (its scripts, styles and icons). */
+function referencedAssets(html) {
+  const out = new Set();
+  for (const m of html.matchAll(/(?:src|href)="(\/[^"]+\.(?:js|css|svg|png|ico|woff2?))"/g)) {
+    out.add(m[1]);
+  }
+  return [...out];
+}
+
+/** Keep the app page, and everything it loads, for opening without a connection. */
+async function saveApp(res) {
+  const html = await res.text();
+  const cache = await caches.open(SHELL);
+  const previous = await cache.match(APP_KEY);
+  if (previous && (await previous.text()) === html) return;
+  await cache.put(APP_KEY, new Response(html, { headers: { "Content-Type": "text/html" } }));
+  const wanted = referencedAssets(html);
+  // A new release: its files have new names, so the old ones can go. Files
+  // loaded later (lazy chunks) are saved again the first time they are used.
+  for (const req of await cache.keys()) {
+    const p = new URL(req.url).pathname;
+    if (p.startsWith("/assets/") && !wanted.includes(p)) await cache.delete(req);
+  }
+  await Promise.all(
+    wanted.map(async (p) => {
+      if (await cache.match(p)) return;
+      try {
+        const r = await fetch(p);
+        if (r.ok) await cache.put(p, r);
+      } catch {
+        /* next time */
+      }
+    }),
+  );
+}
+
+async function appPage(event, req) {
+  try {
+    const res = await network(req, offline ? RETRY_TIMEOUT : NAV_TIMEOUT);
+    offline = false;
+    if (res.ok && (res.headers.get("Content-Type") || "").includes("text/html")) {
+      event.waitUntil(saveApp(res.clone()).catch(() => {}));
+    }
+    return res;
+  } catch {
+    offline = true;
+    const hit = await (await caches.open(SHELL)).match(APP_KEY);
+    return hit || Response.error();
+  }
+}
+
+/** Hashed build files never change, so a saved copy is always right. */
+async function staticFile(req) {
+  const cache = await caches.open(SHELL);
+  const hit = await cache.match(req, { ignoreVary: true });
+  if (hit) return hit;
+  const res = await fetch(req);
+  if (res.ok) await cache.put(req, res.clone());
+  return res;
+}
+
+const STATIC_RE = /^(?:\/assets\/|\/fonts\/|\/[^/]+\.(?:svg|png|ico)$)/;
+
+function isAppPage(url) {
+  const p = url.pathname;
+  return !p.startsWith("/api/") && !p.startsWith("/admin") && !(p.endsWith(".html") && p !== "/index.html");
+}
+
+self.addEventListener("fetch", (event) => {
+  const req = event.request;
+  if (req.method !== "GET") return;
+  const url = new URL(req.url);
+  if (url.origin !== self.location.origin) return;
+  if (req.mode === "navigate") {
+    if (isAppPage(url)) event.respondWith(appPage(event, req));
+    return;
+  }
+  if (url.pathname.startsWith("/api/")) {
+    if (url.pathname === "/api/ws") return;
+    const cls = classify(url);
+    event.respondWith(cls ? apiData(event, req, cls) : apiOther(event, req));
+    return;
+  }
+  if (STATIC_RE.test(url.pathname)) event.respondWith(staticFile(req));
+});
+
+self.addEventListener("message", (event) => {
+  if (event.data?.type === "purge") event.waitUntil(purge());
+});
+
+self.addEventListener("install", (event) => {
+  // Save the app straight away, so it opens offline even if the page that
+  // turned this on is never reloaded.
+  event.waitUntil(
+    (async () => {
+      try {
+        const res = await fetch("/", { cache: "no-store" });
+        if (res.ok) await saveApp(res);
+      } catch {
+        /* saved on the next visit */
+      }
+      await self.skipWaiting();
+    })(),
+  );
+});
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    (async () => {
+      for (const name of await caches.keys()) {
+        if (name.startsWith("wm-") && name !== SHELL && name !== DATA) await caches.delete(name);
+      }
+      await self.clients.claim();
+    })(),
+  );
+});
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 7ac4235..2f6140d 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -2,6 +2,9 @@ import { useQueryClient } from "@tanstack/react-query";
 import { useEffect } from "react";
 import { Navigate, Route, Routes, useLocation } from "react-router-dom";
 
+import { OfflineController } from "./components/Offline";
+import { useT } from "./i18n";
+import { isOfflineError } from "./lib/api";
 import { useMe } from "./lib/auth";
 import { connectWs } from "./lib/ws";
 import { PluginPage } from "./plugins/PluginPage";
@@ -14,7 +17,8 @@ import { ResetPassword } from "./routes/ResetPassword";
 import { Settings } from "./routes/settings/Settings";
 
 function RequireAuth({ children }: { children: JSX.Element }) {
-  const { data, isLoading, isError } = useMe();
+  const { data, isLoading, isError, error, refetch } = useMe();
+  const t = useT();
   const location = useLocation();
   const queryClient = useQueryClient();
 
@@ -26,10 +30,29 @@ function RequireAuth({ children }: { children: JSX.Element }) {
   if (isLoading) {
     return <div className="p-8 text-gray-500">Loading…</div>;
   }
+  if (isError && isOfflineError(error)) {
+    // No connection and nothing saved on this device: not a reason to sign out.
+    return (
+      <div className="flex h-screen flex-col items-center justify-center gap-3 bg-app-bg text-app-muted">
+        <span>{t("offline.cantReach")}</span>
+        <button
+          className="rounded border border-app-border px-3 py-1.5 text-sm text-app-fg hover:bg-app-hover"
+          onClick={() => void refetch()}
+        >
+          {t("common.retry")}
+        </button>
+      </div>
+    );
+  }
   if (isError || !data) {
     return <Navigate to="/login" state={{ from: location }} replace />;
   }
-  return children;
+  return (
+    <>
+      <OfflineController />
+      {children}
+    </>
+  );
 }
 
 export function App() {
diff --git a/frontend/src/components/Offline.tsx b/frontend/src/components/Offline.tsx
new file mode 100644
index 0000000..9afa293
--- /dev/null
+++ b/frontend/src/components/Offline.tsx
@@ -0,0 +1,264 @@
+// What the app shows and does around being offline:
+//  - OfflineController (once, behind sign-in): notices the connection coming
+//    back, refreshes everything then, keeps the saved mail up to date, and
+//    stops clicks on buttons that need the server (`.wm-online-only`).
+//  - OfflineIndicator (the sidebar header): the amber "Offline" cloud, with an
+//    explanation on click, and a short green "Back online" when it returns.
+
+import { useQueryClient } from "@tanstack/react-query";
+import { useEffect, useRef, useState, useSyncExternalStore } from "react";
+
+import { useT } from "../i18n";
+import {
+  isOffline,
+  offlineEnabled,
+  reportOnline,
+  savedWhen,
+  startOffline,
+  useLastSync,
+  useOffline,
+  useOfflineEnabled,
+} from "../lib/offline";
+import { prefetchRecent } from "../lib/offlinePrefetch";
+import { subscribeWs } from "../lib/ws";
+
+const PROBE_MS = 10_000;
+const BACK_ONLINE_MS = 3_000;
+const REFRESH_MS = 15 * 60_000;
+
+// ---- the short-lived bits of UI the two share ---------------------------------
+
+type Notice = { key: "offline" | "offlineNotSaved" | "notAvailable"; seq: number } | null;
+
+let notice: Notice = null;
+let backOnline = false;
+const listeners = new Set<() => void>();
+const emit = () => listeners.forEach((l) => l());
+const subscribe = (l: () => void) => {
+  listeners.add(l);
+  return () => {
+    listeners.delete(l);
+  };
+};
+let noticeTimer: number | undefined;
+let noticeSeq = 0;
+
+function showNotice(key: NonNullable<Notice>["key"], ms: number) {
+  notice = { key, seq: ++noticeSeq };
+  emit();
+  window.clearTimeout(noticeTimer);
+  noticeTimer = window.setTimeout(() => {
+    notice = null;
+    emit();
+  }, ms);
+}
+
+function CloudOff({ size = 16 }: { size?: number }) {
+  return (
+    <svg width={size} height={size} viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.9" strokeLinecap="round" strokeLinejoin="round" aria-hidden>
+      <path d="M2 2l20 20" />
+      <path d="M5.8 5.8A7 7 0 0 0 9 19h8.5a4.4 4.4 0 0 0 1.2-.17" />
+      <path d="M21.5 15.5A4.5 4.5 0 0 0 17.5 10h-1.3A7 7 0 0 0 9.4 5.1" />
+    </svg>
+  );
+}
+
+function Check({ size = 16 }: { size?: number }) {
+  return (
+    <svg width={size} height={size} viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.2" strokeLinecap="round" strokeLinejoin="round" aria-hidden>
+      <path d="M5 12.5l4.5 4.5L19 7.5" />
+    </svg>
+  );
+}
+
+// ---- the controller ------------------------------------------------------------
+
+export function OfflineController() {
+  const t = useT();
+  const queryClient = useQueryClient();
+  const offline = useOffline();
+  const enabled = useOfflineEnabled();
+  const current = useSyncExternalStore(subscribe, () => notice);
+  const wasOffline = useRef(false);
+
+  // Install (or remove) the service worker to match this device's setting.
+  useEffect(() => {
+    void startOffline();
+  }, [enabled]);
+
+  // Going offline: say so once; coming back: refresh everything and save anew.
+  useEffect(() => {
+    if (offline) {
+      wasOffline.current = true;
+      showNotice(offlineEnabled() ? "offline" : "offlineNotSaved", 6000);
+      return;
+    }
+    if (!wasOffline.current) return;
+    wasOffline.current = false;
+    notice = null;
+    backOnline = true;
+    emit();
+    const timer = window.setTimeout(() => {
+      backOnline = false;
+      emit();
+    }, BACK_ONLINE_MS);
+    void queryClient.invalidateQueries();
+    void prefetchRecent();
+    return () => window.clearTimeout(timer);
+  }, [offline, queryClient]);
+
+  // While offline: ask the server now and then whether it is back.
+  useEffect(() => {
+    if (!offline) return;
+    const probe = async () => {
+      try {
+        const resp = await fetch("/api/healthz", { cache: "no-store", credentials: "include" });
+        if (!resp.headers.has("X-WM-Offline")) reportOnline();
+      } catch {
+        /* still offline */
+      }
+    };
+    const timer = window.setInterval(probe, PROBE_MS);
+    window.addEventListener("online", probe);
+    return () => {
+      window.clearInterval(timer);
+      window.removeEventListener("online", probe);
+    };
+  }, [offline]);
+
+  // Keep the saved mail current: shortly after start, after new mail (gathered
+  // up for a minute), and every quarter of an hour.
+  useEffect(() => {
+    if (!enabled) return;
+    const start = window.setTimeout(() => void prefetchRecent(), 5000);
+    const every = window.setInterval(() => void prefetchRecent(), REFRESH_MS);
+    let pending: number | undefined;
+    const off = subscribeWs((event) => {
+      if (event !== "message.new" || pending) return;
+      pending = window.setTimeout(() => {
+        pending = undefined;
+        void prefetchRecent();
+      }, 60_000);
+    });
+    return () => {
+      window.clearTimeout(start);
+      window.clearInterval(every);
+      window.clearTimeout(pending);
+      off();
+    };
+  }, [enabled]);
+
+  // Buttons that need the server do nothing offline, and say why.
+  useEffect(() => {
+    const onClick = (e: MouseEvent) => {
+      if (!isOffline()) return;
+      const el = (e.target as Element | null)?.closest?.(".wm-online-only");
+      if (!el) return;
+      e.preventDefault();
+      e.stopPropagation();
+      showNotice("notAvailable", 2500);
+    };
+    document.addEventListener("click", onClick, true);
+    return () => document.removeEventListener("click", onClick, true);
+  }, []);
+
+  if (!current) return null;
+  return (
+    <div
+      key={current.seq}
+      role="status"
+      className="wm-offline-notice pointer-events-none fixed bottom-6 left-1/2 z-[70] flex max-w-[calc(100vw-2rem)] -translate-x-1/2 items-center gap-2.5 rounded-xl border border-app-border bg-app-elevated px-4 py-2.5 text-sm text-app-fg shadow-lg"
+    >
+      <span className="text-amber-500">
+        <CloudOff size={18} />
+      </span>
+      {current.key === "offline"
+        ? t("offline.noticeSaved")
+        : current.key === "offlineNotSaved"
+          ? t("offline.noticeNotSaved")
+          : t("offline.notAvailable")}
+    </div>
+  );
+}
+
+// ---- the sidebar indicator -----------------------------------------------------
+
+export function OfflineIndicator({ compact = false }: { compact?: boolean }) {
+  const t = useT();
+  const offline = useOffline();
+  const enabled = useOfflineEnabled();
+  const back = useSyncExternalStore(subscribe, () => backOnline);
+  const last = useLastSync();
+  const when = savedWhen(last);
+  const [open, setOpen] = useState(false);
+  const ref = useRef<HTMLDivElement>(null);
+
+  useEffect(() => {
+    if (!offline) setOpen(false);
+  }, [offline]);
+  useEffect(() => {
+    if (!open) return;
+    const onDoc = (e: MouseEvent) => {
+      if (!ref.current?.contains(e.target as Node)) setOpen(false);
+    };
+    document.addEventListener("mousedown", onDoc);
+    return () => document.removeEventListener("mousedown", onDoc);
+  }, [open]);
+
+  if (back && !offline) {
+    return (
+      <span
+        role="status"
+        className="inline-flex h-7 items-center gap-1 rounded px-1.5 text-[13px] font-medium text-emerald-500"
+        title={t("offline.backOnline")}
+      >
+        <Check />
+        {!compact && t("offline.backOnline")}
+      </span>
+    );
+  }
+  if (!offline) return null;
+
+  const explanation = enabled
+    ? when
+      ? t("offline.explainSaved", { when })
+      : t("offline.explainSavedNoTime")
+    : t("offline.explainNotSaved");
+  return (
+    <div ref={ref} className="relative">
+      <button
+        type="button"
+        onClick={() => setOpen((v) => !v)}
+        title={explanation}
+        aria-label={t("offline.label")}
+        aria-expanded={open}
+        className="inline-flex h-7 items-center gap-1 rounded px-1.5 text-[13px] font-medium text-amber-500 hover:bg-app-hover"
+      >
+        <CloudOff />
+        {!compact && t("offline.label")}
+      </button>
+      {open && (
+        <div className="absolute left-0 top-full z-50 mt-1 w-64 rounded-lg border border-app-border bg-app-elevated p-3 text-[13px] leading-snug text-app-fg shadow-lg">
+          <div className="mb-1 flex items-center gap-1.5 font-semibold text-amber-500">
+            <CloudOff size={15} />
+            {t("offline.label")}
+          </div>
+          {explanation}
+        </div>
+      )}
+    </div>
+  );
+}
+
+/** Shown in the reading pane for a conversation that was never saved here. */
+export function NotSavedOffline() {
+  const t = useT();
+  return (
+    <div className="flex h-full flex-col items-center justify-center gap-3 px-6 text-center text-app-faint">
+      <span className="text-amber-500">
+        <CloudOff size={40} />
+      </span>
+      <span className="text-xl">{t("offline.threadNotSaved")}</span>
+    </div>
+  );
+}
diff --git a/frontend/src/components/list/MessageList.tsx b/frontend/src/components/list/MessageList.tsx
index 2090634..628ca96 100644
--- a/frontend/src/components/list/MessageList.tsx
+++ b/frontend/src/components/list/MessageList.tsx
@@ -4,6 +4,7 @@ import { useEffect, useRef } from "react";
 
 import { useT } from "../../i18n";
 import { api } from "../../lib/api";
+import { useSavedThreads } from "../../lib/offline";
 import { formatClock, getDateLocale } from "../../lib/timeFormat";
 import type { Address, ThreadSummary } from "../../lib/types";
 import { Icon } from "../Icon";
@@ -137,6 +138,8 @@ export function MessageList({
     placeholderData: keepPreviousData,
   });
   const labelMap = labelsQuery.data ?? {};
+  // Offline: conversations not saved on this device are shown faded.
+  const saved = useSavedThreads();
 
   // The user's own addresses, shown as "me" in the sender column.
   const accounts = useQuery({ queryKey: ["accounts"], queryFn: api.accounts });
@@ -170,6 +173,7 @@ export function MessageList({
           const selected = selectedId === thread.id;
           const checked = checkedIds.has(thread.id);
           const dimmed = dimmedIds?.has(thread.id) ?? false;
+          const unsaved = saved !== null && !saved.has(thread.id);
           // The sender column lists everyone who wrote in the thread, oldest first,
           // with the user's own accounts shown as "me" — so a thread you started
           // and someone answered reads "me, Alice". Recipients are not listed.
@@ -206,7 +210,7 @@ export function MessageList({
               // A row leaving the list lets the ones below glide up into its place.
               className={`group absolute inset-x-0 px-2 ${compact ? "pt-5" : "pt-[7px]"} motion-safe:transition-transform motion-safe:duration-200 motion-safe:ease-out ${
                 dimmed ? "pointer-events-none opacity-40" : "cursor-pointer"
-              }`}
+              } ${unsaved ? "opacity-45" : ""}`}
             >
               <div
                 className={`flex h-full items-center gap-3 rounded-lg px-3 ${
@@ -355,8 +359,8 @@ export function MessageList({
                     onClick={stop(() => onTogglePin(thread.id, thread.pinned))}
                     className={
                       thread.pinned
-                        ? "text-[rgb(220_129_143)]"
-                        : "invisible text-app-muted hover:text-app-fg group-hover:visible"
+                        ? "wm-online-only text-[rgb(220_129_143)]"
+                        : "wm-online-only invisible text-app-muted hover:text-app-fg group-hover:visible"
                     }
                   >
                     <Icon name="pin" size={17} />
@@ -368,7 +372,7 @@ export function MessageList({
                       type="button"
                       title={t("mail.restore")}
                       onClick={stop(() => onRestore(thread.id))}
-                      className="invisible text-app-muted hover:text-app-fg group-hover:visible"
+                      className="wm-online-only invisible text-app-muted hover:text-app-fg group-hover:visible"
                     >
                       <Icon name="restore" size={17} />
                     </button>
@@ -377,7 +381,7 @@ export function MessageList({
                       type="button"
                       title={t("mail.archive")}
                       onClick={stop(() => onArchive(thread.id))}
-                      className="invisible text-app-muted hover:text-app-fg group-hover:visible"
+                      className="wm-online-only invisible text-app-muted hover:text-app-fg group-hover:visible"
                     >
                       <Icon name="archive" size={15} />
                     </button>
@@ -388,7 +392,7 @@ export function MessageList({
                     onClick={stop(() =>
                       folder === "trash" ? onDeletePermanent(thread.id) : onTrash(thread.id),
                     )}
-                    className="invisible mr-[7px] text-app-muted hover:text-app-fg group-hover:visible"
+                    className="wm-online-only invisible mr-[7px] text-app-muted hover:text-app-fg group-hover:visible"
                   >
                     <Icon name="trash" size={17} />
                   </button>
diff --git a/frontend/src/components/thread/MessageBody.tsx b/frontend/src/components/thread/MessageBody.tsx
index 8aaad7a..d59f228 100644
--- a/frontend/src/components/thread/MessageBody.tsx
+++ b/frontend/src/components/thread/MessageBody.tsx
@@ -30,13 +30,14 @@ export function MessageBody({
   onMailtoRef.current = onMailto;
   const [height, setHeight] = useState(80);
 
-  const { data, isLoading, isError } = useQuery({
+  const { data, isLoading, isError, error } = useQuery({
     queryKey: ["message-html", message.id, allowed],
     queryFn: async () => {
       // ?images=1 asks the server to render remote images for this one request
       // (the "Load images" one-off); "Always from this sender" persists separately.
       const url = `/api/messages/${message.id}/html${allowed ? "?images=1" : ""}`;
       const resp = await fetch(url, { credentials: "include" });
+      if (resp.headers.has("X-WM-Offline") && !resp.ok) throw new Error("offline");
       if (!resp.ok) throw new Error("failed to load message");
       return resp.text();
     },
@@ -170,7 +171,11 @@ export function MessageBody({
         </div>
       )}
       {isLoading && <div className="p-3 text-sm text-app-faint">{t("mail.loadingThread")}</div>}
-      {isError && <div className="p-3 text-sm text-red-500">{t("mail.loadFailed")}</div>}
+      {isError && (
+        <div className={`p-3 text-sm ${error.message === "offline" ? "text-app-faint" : "text-red-500"}`}>
+          {error.message === "offline" ? t("offline.notAvailable") : t("mail.loadFailed")}
+        </div>
+      )}
       {data !== undefined && !isLoading && (
         <iframe
           ref={frameRef}
diff --git a/frontend/src/components/thread/ThreadView.tsx b/frontend/src/components/thread/ThreadView.tsx
index fd00123..afe95e4 100644
--- a/frontend/src/components/thread/ThreadView.tsx
+++ b/frontend/src/components/thread/ThreadView.tsx
@@ -211,7 +211,7 @@ function ActionsMenu({
   return (
     <div ref={ref} className="relative">
       <button
-        className="flex items-center gap-1 rounded px-1.5 py-1 text-xs text-app-muted hover:bg-app-hover hover:text-app-fg"
+        className="wm-online-only flex items-center gap-1 rounded px-1.5 py-1 text-xs text-app-muted hover:bg-app-hover hover:text-app-fg"
         onClick={() => setOpen((v) => !v)}
       >
         {t("mail.actions")}
@@ -343,7 +343,7 @@ function MessageItem({
   const [showDetails, setShowDetails] = useState(false);
   const replyBtn = (kind: string, label: string, icon: string) => (
     <button
-      className="flex items-center gap-1.5 rounded-md border border-app-border px-3 py-1.5 text-xs text-app-fg hover:bg-app-hover"
+      className="wm-online-only flex items-center gap-1.5 rounded-md border border-app-border px-3 py-1.5 text-xs text-app-fg hover:bg-app-hover"
       onClick={() => onReply(kind)}
     >
       <Icon name={icon} size={14} />
@@ -430,7 +430,7 @@ function MessageItem({
                     title={t("mail.markReadUnread")}
                     aria-label={t("mail.markReadUnread")}
                     onClick={onToggleRead}
-                    className="grid h-4 w-4 place-items-center"
+                    className="wm-online-only grid h-4 w-4 place-items-center"
                   >
                     <span
                       className={`h-3 w-3 rounded-full border ${
@@ -442,7 +442,7 @@ function MessageItem({
                   </button>
                   {canReply && (
                     <button
-                      className="flex items-center gap-1 text-app-muted hover:text-app-fg"
+                      className="wm-online-only flex items-center gap-1 text-app-muted hover:text-app-fg"
                       onClick={() => onReply(multiRecipient ? "reply_all" : "reply")}
                     >
                       <Icon name={multiRecipient ? "reply-all" : "reply"} size={14} />
diff --git a/frontend/src/i18n/en.ts b/frontend/src/i18n/en.ts
index 1b4fe30..26dbad7 100644
--- a/frontend/src/i18n/en.ts
+++ b/frontend/src/i18n/en.ts
@@ -545,6 +545,27 @@ export const en = {
   "theme.dark": "Dark",
   "theme.system": "System",
   "theme.toggle": "Toggle theme",
+  "offline.label": "Offline",
+  "offline.backOnline": "Back online",
+  "offline.explainSaved": "Offline — showing mail saved on this device, last updated {when}.",
+  "offline.explainSavedNoTime": "Offline — showing mail saved on this device.",
+  "offline.explainNotSaved":
+    "Can't reach the server. To read recent mail without a connection, turn on offline access in Settings → Security.",
+  "offline.noticeSaved": "You're offline. Recent mail is still available.",
+  "offline.noticeNotSaved": "You're offline — can't reach the server.",
+  "offline.notAvailable": "Not available offline",
+  "offline.threadNotSaved": "This conversation isn't saved on this device.",
+  "offline.folderNotSaved": "This folder isn't saved on this device.",
+  "offline.searchPlaceholder": "Search (offline)",
+  "offline.cantReach": "Can't reach the server.",
+  "offline.settingTitle": "Offline access",
+  "offline.settingHelp":
+    "Keep your newest 50 Inbox conversations, and the ones you open, on this device, so you can read them without a connection. They are stored unencrypted in this browser and removed when you sign out — leave this off on shared computers.",
+  "offline.settingStatus": "{count} conversations saved · {size} · updated {when}",
+  "offline.settingStatusNoTime": "{count} conversations saved · {size}",
+  "offline.settingSaving": "Saving recent mail…",
+  "offline.settingUnsupported":
+    "Not available here: browsers allow offline access only when the app is opened over HTTPS.",
 } as const;
 
 export type MessageKey = keyof typeof en;
diff --git a/frontend/src/index.css b/frontend/src/index.css
index 6b4121d..d39f4b2 100644
--- a/frontend/src/index.css
+++ b/frontend/src/index.css
@@ -221,3 +221,27 @@ body {
   border: 2px solid transparent;
   background-clip: content-box;
 }
+
+/* Offline (components/Offline.tsx): buttons that need the server look
+   unavailable, and their clicks are stopped with a "Not available offline". */
+html[data-offline] .wm-online-only {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+html[data-offline] .wm-online-only:hover {
+  background-color: transparent;
+}
+
+.wm-offline-notice {
+  animation: wm-offline-in 180ms ease-out;
+}
+@keyframes wm-offline-in {
+  from {
+    opacity: 0;
+    transform: translate(-50%, 8px);
+  }
+  to {
+    opacity: 1;
+    transform: translate(-50%, 0);
+  }
+}
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index 73c6309..e6417d3 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -32,6 +32,12 @@ import type {
   UserSettings,
 } from "./types";
 import type { UiManifest } from "../plugins/types";
+import { noteResponse, reportOffline } from "./offline";
+
+/** The request failed only because the server could not be reached. */
+export function isOfflineError(error: unknown): boolean {
+  return error instanceof ApiError && error.code === "offline";
+}
 
 export class ApiError extends Error {
   code: string;
@@ -65,12 +71,21 @@ export async function apiFetch<T>(path: string, init: RequestInit = {}): Promise
       headers.set("Content-Type", "application/json");
     }
   }
-  const resp = await fetch(`/api${path}`, {
-    ...init,
-    method,
-    headers,
-    credentials: "include",
-  });
+  let resp: Response;
+  try {
+    resp = await fetch(`/api${path}`, {
+      ...init,
+      method,
+      headers,
+      credentials: "include",
+    });
+  } catch (err) {
+    if (err instanceof DOMException && err.name === "AbortError") throw err;
+    // The server could not be reached at all (and nothing saved stood in).
+    reportOffline();
+    throw new ApiError(0, "offline", "Not available offline");
+  }
+  noteResponse(resp);
   if (resp.status === 204) return undefined as T;
   const isJson = resp.headers.get("content-type")?.includes("application/json");
   const data = isJson ? await resp.json() : await resp.text();
diff --git a/frontend/src/lib/auth.ts b/frontend/src/lib/auth.ts
index 10488d9..abfb564 100644
--- a/frontend/src/lib/auth.ts
+++ b/frontend/src/lib/auth.ts
@@ -1,6 +1,7 @@
 import { type QueryClient, useQuery } from "@tanstack/react-query";
 
 import { api, ApiError } from "./api";
+import { clearOfflineData } from "./offline";
 
 export function useMe() {
   return useQuery({
@@ -8,7 +9,7 @@ export function useMe() {
     queryFn: api.me,
     retry: (count, error) => {
       // Do not retry an unauthenticated response.
-      if (error instanceof ApiError && error.status === 401) return false;
+      if (error instanceof ApiError && (error.status === 401 || error.code === "offline")) return false;
       return count < 2;
     },
   });
@@ -24,6 +25,7 @@ export function signedOut(queryClient: QueryClient) {
   // "me" first, in place: its watchers see it and route to sign-in. Clearing the
   // whole cache first would orphan them from the query they watch.
   queryClient.setQueryData(["me"], null);
+  void clearOfflineData(); // mail saved for offline reading goes with the session
   queryClient.removeQueries({ predicate: (q) => q.queryKey[0] !== "me" });
 }
 
diff --git a/frontend/src/lib/offline.ts b/frontend/src/lib/offline.ts
new file mode 100644
index 0000000..e8659aa
--- /dev/null
+++ b/frontend/src/lib/offline.ts
@@ -0,0 +1,246 @@
+// Offline access: whether the server can be reached right now, and the
+// per-device "keep recent mail on this device" setting that installs the
+// service worker (public/sw.js) which saves mail for reading without a
+// connection.
+//
+// The offline state is learned from requests themselves: a request that could
+// not reach the server, or an answer the service worker served from its saved
+// copy (marked `X-WM-Offline`), means offline; any answer from the server means
+// online. While offline, OfflineController probes the server to notice the
+// connection coming back.
+
+import { useEffect, useState, useSyncExternalStore } from "react";
+
+import { formatClock, getDateLocale } from "./timeFormat";
+
+const ENABLED_KEY = "offline.enabled";
+const LAST_SYNC_KEY = "offline.lastSync";
+const DATA_CACHE = "wm-data-v1";
+const INDEX_KEY = "/__wm/offline-index";
+
+// ---- online / offline ----------------------------------------------------------
+
+let offline = false;
+const listeners = new Set<() => void>();
+
+function emit() {
+  for (const l of listeners) l();
+}
+
+function subscribe(listener: () => void): () => void {
+  listeners.add(listener);
+  return () => {
+    listeners.delete(listener);
+  };
+}
+
+export function reportOffline(): void {
+  if (offline) return;
+  offline = true;
+  document.documentElement.dataset.offline = "";
+  emit();
+}
+
+export function reportOnline(): void {
+  if (!offline) return;
+  offline = false;
+  delete document.documentElement.dataset.offline;
+  emit();
+}
+
+export function isOffline(): boolean {
+  return offline;
+}
+
+export function useOffline(): boolean {
+  return useSyncExternalStore(subscribe, isOffline, () => false);
+}
+
+/** A response's verdict: served from the saved copy, or by the server. */
+export function noteResponse(resp: Response): void {
+  if (resp.headers.has("X-WM-Offline")) reportOffline();
+  else reportOnline();
+}
+
+// ---- the per-device setting ----------------------------------------------------
+
+/** Whether this browser can keep mail at all (service workers need HTTPS). */
+export function offlineSupported(): boolean {
+  return "serviceWorker" in navigator && window.isSecureContext && "caches" in window;
+}
+
+export function offlineEnabled(): boolean {
+  try {
+    return localStorage.getItem(ENABLED_KEY) === "1";
+  } catch {
+    return false;
+  }
+}
+
+const settingListeners = new Set<() => void>();
+
+function subscribeSetting(listener: () => void): () => void {
+  settingListeners.add(listener);
+  return () => {
+    settingListeners.delete(listener);
+  };
+}
+
+function emitSetting() {
+  for (const l of settingListeners) l();
+}
+
+export function useOfflineEnabled(): boolean {
+  return useSyncExternalStore(subscribeSetting, offlineEnabled, () => false);
+}
+
+/** When mail was last saved for offline reading (ms), or null. */
+export function lastSync(): number | null {
+  try {
+    const v = Number(localStorage.getItem(LAST_SYNC_KEY));
+    return v > 0 ? v : null;
+  } catch {
+    return null;
+  }
+}
+
+export function useLastSync(): number | null {
+  return useSyncExternalStore(subscribeSetting, lastSync, () => null);
+}
+
+/** "14:32" today, "3 Oct, 14:32" before. */
+export function savedWhen(ms: number | null): string {
+  if (!ms) return "";
+  const d = new Date(ms);
+  const sameDay = d.toDateString() === new Date().toDateString();
+  if (sameDay) return formatClock(d);
+  return `${d.toLocaleDateString(getDateLocale(), { day: "numeric", month: "short" })}, ${formatClock(d)}`;
+}
+
+export function setLastSync(ms: number): void {
+  try {
+    localStorage.setItem(LAST_SYNC_KEY, String(ms));
+  } catch {
+    /* fine: only shown in the offline notes */
+  }
+  emitSetting();
+}
+
+/** Install the service worker, if this device keeps mail (call on app start). */
+export async function startOffline(): Promise<void> {
+  if (!("serviceWorker" in navigator)) return;
+  if (offlineEnabled() && offlineSupported()) {
+    try {
+      await navigator.serviceWorker.register("/sw.js", { scope: "/" });
+    } catch (err) {
+      console.warn("offline access: service worker not installed", err);
+    }
+    return;
+  }
+  // Turned off (perhaps in another tab): make sure nothing is left behind.
+  const regs = await navigator.serviceWorker.getRegistrations();
+  if (regs.length) await disableOffline();
+}
+
+/** Resolves once the service worker controls this page (it saves what we fetch). */
+export async function offlineReady(): Promise<boolean> {
+  if (!("serviceWorker" in navigator) || !offlineEnabled()) return false;
+  await navigator.serviceWorker.ready;
+  if (navigator.serviceWorker.controller) return true;
+  return new Promise((resolve) => {
+    const timer = window.setTimeout(() => resolve(false), 5000);
+    navigator.serviceWorker.addEventListener(
+      "controllerchange",
+      () => {
+        window.clearTimeout(timer);
+        resolve(true);
+      },
+      { once: true },
+    );
+  });
+}
+
+export async function enableOffline(): Promise<void> {
+  localStorage.setItem(ENABLED_KEY, "1");
+  emitSetting();
+  await startOffline();
+}
+
+export async function disableOffline(): Promise<void> {
+  try {
+    localStorage.removeItem(ENABLED_KEY);
+    localStorage.removeItem(LAST_SYNC_KEY);
+  } catch {
+    /* ignore */
+  }
+  emitSetting();
+  if ("serviceWorker" in navigator) {
+    for (const reg of await navigator.serviceWorker.getRegistrations()) await reg.unregister();
+  }
+  if ("caches" in window) {
+    for (const name of await caches.keys()) if (name.startsWith("wm-")) await caches.delete(name);
+  }
+}
+
+/** Forget the saved mail (signed out); the setting itself stays on. */
+export async function clearOfflineData(): Promise<void> {
+  try {
+    localStorage.removeItem(LAST_SYNC_KEY);
+  } catch {
+    /* ignore */
+  }
+  emitSetting();
+  const sw = "serviceWorker" in navigator ? navigator.serviceWorker.controller : null;
+  // Through the service worker when there is one: it keeps an index of what is
+  // saved in memory, which must be reset along with the saved copies.
+  if (sw) sw.postMessage({ type: "purge" });
+  else if ("caches" in window) await caches.delete(DATA_CACHE);
+}
+
+// ---- what is saved -------------------------------------------------------------
+
+interface SavedIndex {
+  threads: Record<string, { at: number; msgs: string[] }>;
+}
+
+/** Ids of the conversations saved on this device (empty when none). */
+export async function savedThreadIds(): Promise<Set<string>> {
+  if (!("caches" in window)) return new Set();
+  try {
+    const cache = await caches.open(DATA_CACHE);
+    const hit = await cache.match(INDEX_KEY);
+    if (!hit) return new Set();
+    const ix = (await hit.json()) as SavedIndex;
+    return new Set(Object.keys(ix.threads ?? {}));
+  } catch {
+    return new Set();
+  }
+}
+
+/** Whether a response is already saved (so pre-loading can skip it). */
+export async function isSaved(url: string): Promise<boolean> {
+  if (!("caches" in window)) return false;
+  try {
+    return Boolean(await (await caches.open(DATA_CACHE)).match(url, { ignoreVary: true }));
+  } catch {
+    return false;
+  }
+}
+
+/** While offline, the conversations saved here (null while online). */
+export function useSavedThreads(): Set<string> | null {
+  const off = useOffline();
+  const [saved, setSaved] = useState<Set<string> | null>(null);
+  useEffect(() => {
+    if (!off) {
+      setSaved(null);
+      return;
+    }
+    let live = true;
+    void savedThreadIds().then((ids) => live && setSaved(ids));
+    return () => {
+      live = false;
+    };
+  }, [off]);
+  return off ? saved : null;
+}
diff --git a/frontend/src/lib/offlinePrefetch.ts b/frontend/src/lib/offlinePrefetch.ts
new file mode 100644
index 0000000..6878e98
--- /dev/null
+++ b/frontend/src/lib/offlinePrefetch.ts
@@ -0,0 +1,73 @@
+// Pre-loading the newest Inbox conversations for offline reading. Everything is
+// fetched exactly as the app would fetch it when opened — same URLs — and the
+// service worker saves the answers as they pass (public/sw.js). Bodies and
+// images are fetched once; the conversation itself is fetched every time,
+// since new replies, flags and moves change it.
+
+import { api } from "./api";
+import { isOffline, isSaved, offlineReady, setLastSync } from "./offline";
+
+const INBOX = "folder:inbox";
+const COUNT = 50; // the list's page size, so the first page is saved as shown
+const PARALLEL = 3;
+const CID_RE = /\/api\/messages\/[0-9a-fA-F-]{36}\/cid\/[^"'\s)&]+/g;
+
+let running: Promise<void> | null = null;
+
+async function inPool<T>(items: T[], n: number, fn: (item: T) => Promise<void>): Promise<void> {
+  let next = 0;
+  const worker = async () => {
+    while (next < items.length && !isOffline()) {
+      const item = items[next++];
+      try {
+        await fn(item);
+      } catch {
+        /* one conversation failing must not stop the rest */
+      }
+    }
+  };
+  await Promise.all(Array.from({ length: n }, worker));
+}
+
+async function fetchOnce(url: string): Promise<string | null> {
+  if (await isSaved(url)) return null;
+  const resp = await fetch(url, { credentials: "include" });
+  return resp.ok ? resp.text() : null;
+}
+
+async function run(): Promise<void> {
+  if (!(await offlineReady()) || isOffline()) return;
+  const settings = await api.settings();
+  const order = settings.thread_order ?? "asc";
+  // The list as the app shows it (newest first is the default), plus the other
+  // order when that is what this device uses, so the list itself opens offline.
+  const list = await api.threads(INBOX, { limit: COUNT, order: "desc" });
+  if (localStorage.getItem("mail.listOrder") === "oldest") {
+    await api.threads(INBOX, { limit: COUNT, order: "asc" });
+  }
+  await api.folderCounts();
+
+  await inPool(list.threads, PARALLEL, async (summary) => {
+    const detail = await api.thread(summary.id, order, INBOX);
+    for (const m of detail.messages) {
+      const html = await fetchOnce(`/api/messages/${m.id}/html${m.images_allowed ? "?images=1" : ""}`);
+      if (html) {
+        for (const cid of new Set(html.match(CID_RE) ?? [])) await fetchOnce(cid);
+      }
+      if (m.has_attachments && !(await isSaved(`/api/messages/${m.id}/attachments`))) {
+        await api.messageAttachments(m.id);
+      }
+    }
+  });
+  if (!isOffline()) setLastSync(Date.now());
+}
+
+/** Save the newest Inbox conversations; a no-op unless offline access is on. */
+export function prefetchRecent(): Promise<void> {
+  running ??= run()
+    .catch(() => {})
+    .finally(() => {
+      running = null;
+    });
+  return running;
+}
diff --git a/frontend/src/routes/MailLayout.tsx b/frontend/src/routes/MailLayout.tsx
index 5f5e630..b3054dc 100644
--- a/frontend/src/routes/MailLayout.tsx
+++ b/frontend/src/routes/MailLayout.tsx
@@ -24,12 +24,14 @@ import { Resizer } from "../components/Resizer";
 import { ThemeToggle } from "../components/ThemeToggle";
 import { EmptyIllustration } from "../components/EmptyIllustration";
 import { BackfillBanner } from "../components/BackfillBanner";
+import { NotSavedOffline, OfflineIndicator } from "../components/Offline";
 import { ThreadView } from "../components/thread/ThreadView";
 import { MailboxDisplay } from "../components/mail/MailboxDisplay";
 import { RightPanel, type PanelTab } from "../components/mail/RightPanel";
 import { useT } from "../i18n";
 import type { MessageKey } from "../i18n/en";
-import { api } from "../lib/api";
+import { api, isOfflineError } from "../lib/api";
+import { clearOfflineData, useOffline } from "../lib/offline";
 import { setTimeFormat } from "../lib/timeFormat";
 import type { QuotaOut, SavedSearch, ThreadSummary } from "../lib/types";
 import { useReadingPane } from "../lib/readingPane";
@@ -71,6 +73,7 @@ export function MailLayout() {
   const match = useMatch("/mail/t/:threadId");
   const threadId = match?.params.threadId;
   const [view, setView] = useState<string>("folder:inbox");
+  const offline = useOffline();
   // Smartphone layout: one column at a time — the list, or the reading pane full
   // screen — with the sidebar as a slide-in drawer. Desktop is untouched by it.
   const mobile = useIsMobile();
@@ -789,12 +792,13 @@ export function MailLayout() {
               <Icon name={collapsed ? "show" : "hide"} size={16} />
             </button>
             )}
+            <OfflineIndicator compact={collapsed || mobile} />
             <ThemeToggle />
           </div>
         </div>
         <div className={`pb-2 pt-2.5 ${collapsed ? "px-2" : "px-3"} ${mobile ? "hidden" : ""}`}>
           <button
-            className="flex w-full items-center justify-center gap-2 rounded-lg bg-app-accent py-2 text-[15px] font-medium text-app-accent-fg shadow-sm hover:opacity-90"
+            className="wm-online-only flex w-full items-center justify-center gap-2 rounded-lg bg-app-accent py-2 text-[15px] font-medium text-app-accent-fg shadow-sm hover:opacity-90"
             title={collapsed ? t("nav.compose") : undefined}
             onClick={() => accounts[0] && openComposer({ accountId: accounts[0].id, kind: "new" })}
           >
@@ -903,12 +907,13 @@ export function MailLayout() {
             {!collapsed && t("nav.settings")}
           </Link>
           <button
-            className={`flex w-full items-center gap-2.5 rounded-lg py-2 text-left text-app-muted hover:bg-app-hover hover:text-app-fg ${
+            className={`wm-online-only flex w-full items-center gap-2.5 rounded-lg py-2 text-left text-app-muted hover:bg-app-hover hover:text-app-fg ${
               collapsed ? "justify-center px-0" : "px-3"
             }`}
             title={collapsed ? t("common.signOut") : undefined}
             onClick={async () => {
               await api.logout();
+              await clearOfflineData();
               queryClient.clear();
               navigate("/login");
             }}
@@ -973,8 +978,11 @@ export function MailLayout() {
               className={`w-full rounded-lg border border-app-border bg-app-bg px-3 py-1.5 text-[15px] text-app-fg placeholder:text-app-faint focus:border-app-accent focus:outline-none ${
                 term ? "pr-14" : "pr-8"
               }`}
+              disabled={offline}
               placeholder={
-                currentFolder === "trash"
+                offline
+                  ? t("offline.searchPlaceholder")
+                  : currentFolder === "trash"
                   ? t("nav.searchInTrash")
                   : currentFolder === "junk"
                     ? t("nav.searchInSpam")
@@ -1077,7 +1085,7 @@ export function MailLayout() {
             <div className="-mr-1.5 ml-auto flex items-center gap-1">
               {(currentFolder === "trash" || currentFolder === "junk") && (
                 <button
-                  className="rounded px-2 py-1 text-[13px] font-medium text-red-500 hover:bg-app-hover disabled:opacity-50"
+                  className="wm-online-only rounded px-2 py-1 text-[13px] font-medium text-red-500 hover:bg-app-hover disabled:opacity-50"
                   onClick={emptyCurrentFolder}
                   disabled={emptying}
                 >
@@ -1148,7 +1156,9 @@ export function MailLayout() {
           </div>
         )}
         <div className="min-h-0 flex-1">
-          {isDrafts && !term.trim() ? (
+          {isDrafts && !term.trim() && !draftsQuery.data && isOfflineError(draftsQuery.error) ? (
+            <div className="p-6 text-[15px] text-app-faint">{t("offline.folderNotSaved")}</div>
+          ) : isDrafts && !term.trim() ? (
             <DraftsList
               drafts={draftsQuery.data ?? []}
               remoteDrafts={remoteDraftsQuery.data ?? []}
@@ -1162,6 +1172,8 @@ export function MailLayout() {
                 openDraftInPane(draftToInit(draft));
               }}
             />
+          ) : !searching && !threadsQuery.data && isOfflineError(threadsQuery.error) ? (
+            <div className="p-6 text-[15px] text-app-faint">{t("offline.folderNotSaved")}</div>
           ) : (
             <MessageList
               threads={pageThreads}
@@ -1190,7 +1202,7 @@ export function MailLayout() {
           }`}
         >
           <button
-            className={`-ml-1 rounded text-app-muted ${mobile ? "p-1.5" : "p-1"} hover:bg-app-hover hover:text-app-fg disabled:opacity-60`}
+            className={`wm-online-only -ml-1 rounded text-app-muted ${mobile ? "p-1.5" : "p-1"} hover:bg-app-hover hover:text-app-fg disabled:opacity-60`}
             title={t("nav.refresh")}
             aria-label={t("nav.refresh")}
             onClick={doRefresh}
@@ -1230,7 +1242,7 @@ export function MailLayout() {
             title={t("nav.compose")}
             aria-label={t("nav.compose")}
             onClick={() => accounts[0] && openComposer({ accountId: accounts[0].id, kind: "new" })}
-            className="absolute bottom-[calc(4rem+env(safe-area-inset-bottom))] right-4 z-30 flex h-14 w-14 items-center justify-center rounded-2xl bg-app-accent text-app-accent-fg shadow-lg active:opacity-90"
+            className="wm-online-only absolute bottom-[calc(4rem+env(safe-area-inset-bottom))] right-4 z-30 flex h-14 w-14 items-center justify-center rounded-2xl bg-app-accent text-app-accent-fg shadow-lg active:opacity-90"
           >
             <Icon name="compose" size={22} />
           </button>
@@ -1369,6 +1381,27 @@ export function MailLayout() {
               />
             </div>
           </div>
+        ) : threadId && isOfflineError(detailQuery.error) ? (
+          <div className="flex h-full flex-col">
+            {single && (
+              <div className={paneBarClass}>
+                <button
+                  type="button"
+                  title={t("common.back")}
+                  aria-label={t("common.back")}
+                  onClick={() => navigate("/mail")}
+                  className="-ml-1 mr-1 rounded p-1.5 text-app-fg hover:bg-app-hover"
+                >
+                  <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.6" strokeLinecap="round" strokeLinejoin="round" aria-hidden>
+                    <path d="M15 5l-7 7 7 7" />
+                  </svg>
+                </button>
+              </div>
+            )}
+            <div className="min-h-0 flex-1">
+              <NotSavedOffline />
+            </div>
+          </div>
         ) : (
           <div className="flex h-full flex-col items-center justify-center gap-6 text-app-faint">
             {!rightOpen && !panelToggleInList && (
@@ -1599,7 +1632,7 @@ function ToolbarButton({
 }) {
   return (
     <button
-      className={`flex items-center gap-1.5 rounded px-2.5 py-1 hover:bg-app-hover hover:text-app-fg ${
+      className={`wm-online-only flex items-center gap-1.5 rounded px-2.5 py-1 hover:bg-app-hover hover:text-app-fg ${
         active ? "text-app-fg" : "text-app-muted"
       }`}
       onClick={onClick}
diff --git a/frontend/src/routes/settings/Settings.tsx b/frontend/src/routes/settings/Settings.tsx
index eec6dfc..bfbecfd 100644
--- a/frontend/src/routes/settings/Settings.tsx
+++ b/frontend/src/routes/settings/Settings.tsx
@@ -10,6 +10,16 @@ import { useT } from "../../i18n";
 import { api, ApiError } from "../../lib/api";
 import type { CalendarOut, ReminderPref } from "../../lib/types";
 import { useMe } from "../../lib/auth";
+import {
+  disableOffline,
+  enableOffline,
+  offlineSupported,
+  savedThreadIds,
+  savedWhen,
+  useLastSync,
+  useOfflineEnabled,
+} from "../../lib/offline";
+import { prefetchRecent } from "../../lib/offlinePrefetch";
 import { useTheme } from "../../lib/theme";
 import { setTimeFormat } from "../../lib/timeFormat";
 import { PluginSlot } from "../../plugins/PluginSlot";
@@ -213,6 +223,84 @@ function Row({ label, value }: { label: string; value: string }) {
   );
 }
 
+function formatMB(bytes: number): string {
+  return bytes < 1024 * 1024 ? `${Math.max(1, Math.round(bytes / 1024))} KB` : `${(bytes / 1024 / 1024).toFixed(1)} MB`;
+}
+
+/** Settings → Security: keep recent mail on this device for reading offline. */
+function OfflineAccessSetting() {
+  const t = useT();
+  const enabled = useOfflineEnabled();
+  const last = useLastSync();
+  const supported = offlineSupported();
+  const [busy, setBusy] = useState(false);
+  const [status, setStatus] = useState<{ count: number; bytes: number } | null>(null);
+
+  useEffect(() => {
+    if (!enabled) {
+      setStatus(null);
+      return;
+    }
+    let live = true;
+    void (async () => {
+      const ids = await savedThreadIds();
+      const usage = (await navigator.storage?.estimate?.())?.usage ?? 0;
+      if (live) setStatus({ count: ids.size, bytes: usage });
+    })();
+    return () => {
+      live = false;
+    };
+  }, [enabled, last]);
+
+  const toggle = async (on: boolean) => {
+    setBusy(true);
+    try {
+      if (on) {
+        await enableOffline();
+        await prefetchRecent();
+      } else {
+        await disableOffline();
+      }
+    } finally {
+      setBusy(false);
+    }
+  };
+
+  return (
+    <div className="space-y-2">
+      <div className="flex items-center justify-between gap-3">
+        <h2 className="text-lg font-semibold">{t("offline.settingTitle")}</h2>
+        <div className="flex gap-1 rounded-lg border border-app-border p-0.5 text-sm">
+          {([true, false] as const).map((value) => (
+            <button
+              key={String(value)}
+              disabled={!supported || busy}
+              onClick={() => void toggle(value)}
+              className={`rounded px-3 py-1 disabled:opacity-50 ${
+                enabled === value ? "bg-app-active font-medium text-app-fg" : "text-app-muted hover:bg-app-hover"
+              }`}
+            >
+              {t(value ? "settings.on" : "settings.off")}
+            </button>
+          ))}
+        </div>
+      </div>
+      <p className="text-sm text-app-muted">{t("offline.settingHelp")}</p>
+      {!supported ? (
+        <p className="text-xs text-app-faint">{t("offline.settingUnsupported")}</p>
+      ) : busy && enabled ? (
+        <p className="text-xs text-app-faint">{t("offline.settingSaving")}</p>
+      ) : enabled && status ? (
+        <p className="text-xs text-app-faint">
+          {last
+            ? t("offline.settingStatus", { count: status.count, size: formatMB(status.bytes), when: savedWhen(last) })
+            : t("offline.settingStatusNoTime", { count: status.count, size: formatMB(status.bytes) })}
+        </p>
+      ) : null}
+    </div>
+  );
+}
+
 function SecuritySection() {
   const t = useT();
   const queryClient = useQueryClient();
@@ -351,6 +439,7 @@ function SecuritySection() {
           ))}
         </ul>
       </div>
+      <OfflineAccessSetting />
     </section>
   );
 }