diff --git a/frontend/src/components/compose/RecipientChips.tsx b/frontend/src/components/compose/RecipientChips.tsx
index 09e2905..85ef97c 100644
--- a/frontend/src/components/compose/RecipientChips.tsx
+++ b/frontend/src/components/compose/RecipientChips.tsx
@@ -17,6 +17,13 @@ function parseAddr(token: string): { name: string; email: string } {
   return { name: "", email: token };
 }
 
+// Something that can actually be sent to: a bare address or "Name <address>",
+// with a dotted domain. A typed name alone ("John") is not one.
+const EMAIL = /^[^\s@<>,;]+@[^\s@<>,;]+\.[^\s@<>,;]+$/;
+export function isValidRecipient(token: string): boolean {
+  return EMAIL.test(parseAddr(token.trim()).email.trim());
+}
+
 // A recipient field rendering committed addresses as removable chips with an
 // inline autocomplete input (Fastmail-style). The value stays a comma-joined
 // string ("Name <email>" or bare email) so the draft payload is unchanged.
@@ -38,16 +45,34 @@ export function RecipientChips({
   const [draft, setDraft] = useState("");
   const [suggestions, setSuggestions] = useState<RecipientSuggestion[]>([]);
   const [open, setOpen] = useState(false);
+  // Typed text that is not an address, left in the input (red) rather than
+  // turned into a recipient that could never be delivered.
+  const [invalid, setInvalid] = useState(false);
   const boxRef = useRef<HTMLDivElement>(null);
 
-  const commit = (token: string) => {
-    const tok = token.trim().replace(/,+$/, "").trim();
-    if (!tok) return;
-    onChange([...chips, tok].join(", "));
+  // A chosen suggestion is an address by construction.
+  const pick = (token: string) => {
+    onChange([...chips, token].join(", "));
     setDraft("");
+    setInvalid(false);
     setOpen(false);
     setSuggestions([]);
   };
+
+  // Typed text becomes chips only where it is a valid address; a paste of
+  // several ("a@x.com, John") keeps the rest in the input to fix or complete.
+  const commit = (text: string) => {
+    const tokens = splitList(text);
+    const good = tokens.filter(isValidRecipient);
+    const bad = tokens.filter((tok) => !isValidRecipient(tok));
+    if (good.length) onChange([...chips, ...good].join(", "));
+    setDraft(bad.join(", "));
+    setInvalid(bad.length > 0);
+    if (!bad.length) {
+      setOpen(false);
+      setSuggestions([]);
+    }
+  };
   const removeAt = (i: number) => onChange(chips.filter((_, idx) => idx !== i).join(", "));
 
   useEffect(() => {
@@ -106,12 +131,18 @@ export function RecipientChips({
           );
         })}
         <input
-          className="min-w-[8rem] flex-1 bg-transparent py-1 text-sm text-app-fg outline-none placeholder:text-app-faint"
+          className={`min-w-[8rem] flex-1 bg-transparent py-1 text-sm outline-none placeholder:text-app-faint ${
+            invalid ? "text-red-500" : "text-app-fg"
+          }`}
+          aria-invalid={invalid}
           value={draft}
           placeholder={chips.length === 0 ? t("compose.addRecipient") : ""}
           autoFocus={autoFocus}
           autoComplete="off"
-          onChange={(e) => setDraft(e.target.value)}
+          onChange={(e) => {
+            setDraft(e.target.value);
+            setInvalid(false);
+          }}
           onKeyDown={(e) => {
             if (e.key === "Enter" || e.key === ",") {
               e.preventDefault();
@@ -134,7 +165,7 @@ export function RecipientChips({
                 className="flex w-full flex-col px-3 py-1.5 text-left hover:bg-app-hover"
                 onMouseDown={(e) => {
                   e.preventDefault();
-                  commit(s.name ? `${s.name} <${s.email}>` : s.email);
+                  pick(s.name ? `${s.name} <${s.email}>` : s.email);
                 }}
               >
                 {s.name && <span className="text-sm text-app-fg">{s.name}</span>}
diff --git a/frontend/src/test/recipientChips.test.tsx b/frontend/src/test/recipientChips.test.tsx
new file mode 100644
index 0000000..e1e68b7
--- /dev/null
+++ b/frontend/src/test/recipientChips.test.tsx
@@ -0,0 +1,86 @@
+import { fireEvent, render, screen } from "@testing-library/react";
+import { useState } from "react";
+import { describe, expect, it, vi } from "vitest";
+
+import {
+  isValidRecipient,
+  RecipientChips,
+} from "../components/compose/RecipientChips";
+import { I18nProvider } from "../i18n";
+
+vi.mock("../lib/api", () => ({
+  api: { suggestRecipients: vi.fn().mockResolvedValue([]) },
+}));
+
+// A controlled field, reporting every value it is given.
+function Field({ seen }: { seen: string[] }) {
+  const [value, setValue] = useState("");
+  return (
+    <I18nProvider>
+      <RecipientChips
+        label="To"
+        value={value}
+        onChange={(v) => {
+          seen.push(v);
+          setValue(v);
+        }}
+      />
+    </I18nProvider>
+  );
+}
+
+function type(text: string) {
+  const input = screen.getByRole("textbox");
+  fireEvent.change(input, { target: { value: text } });
+  return input as HTMLInputElement;
+}
+
+describe("RecipientChips", () => {
+  it("leaves typed text that is not an address in the input on blur", () => {
+    const seen: string[] = [];
+    render(<Field seen={seen} />);
+    const input = type("John");
+    fireEvent.blur(input);
+    expect(seen).toEqual([]);
+    expect(input.value).toBe("John");
+    expect(input.getAttribute("aria-invalid")).toBe("true");
+  });
+
+  it("does not turn a non-address into a recipient on Enter either", () => {
+    const seen: string[] = [];
+    render(<Field seen={seen} />);
+    const input = type("john@");
+    fireEvent.keyDown(input, { key: "Enter" });
+    expect(seen).toEqual([]);
+    expect(input.value).toBe("john@");
+  });
+
+  it("commits a valid address on blur", () => {
+    const seen: string[] = [];
+    render(<Field seen={seen} />);
+    const input = type("john@example.com");
+    fireEvent.blur(input);
+    expect(seen).toEqual(["john@example.com"]);
+    expect(input.value).toBe("");
+  });
+
+  it("keeps only the invalid part of a pasted list", () => {
+    const seen: string[] = [];
+    render(<Field seen={seen} />);
+    const input = type("a@x.com, John, Bob <bob@y.org>");
+    fireEvent.keyDown(input, { key: "Enter" });
+    expect(seen).toEqual(["a@x.com, Bob <bob@y.org>"]);
+    expect(input.value).toBe("John");
+  });
+});
+
+describe("isValidRecipient", () => {
+  it.each([
+    ["john@example.com", true],
+    ["John Smith <john@example.com>", true],
+    ["John", false],
+    ["john@", false],
+    ["john@localhost", false],
+    ["John <not an address>", false],
+  ])("%s -> %s", (token, ok) => expect(isValidRecipient(token)).toBe(ok));
+});