diff --git a/src/lib/crypto/__tests__/compose-secure.test.ts b/src/lib/crypto/__tests__/compose-secure.test.ts
index dac3936a..ac73ad25 100644
--- a/src/lib/crypto/__tests__/compose-secure.test.ts
+++ b/src/lib/crypto/__tests__/compose-secure.test.ts
@@ -102,6 +102,18 @@ describe('prepareSecureSend', () => {
     expect(new TextDecoder().decode(uploaded!)).not.toMatch(/bcc@example.org/i);
   }, 60_000);
 
+  it('quotes a Reply-To name with a comma so it stays one address', async () => {
+    await importPgpKey(me.privateKey, me.passphrase);
+    let uploaded: Uint8Array | null = null;
+    const msg = { ...outgoing(), replyTo: [{ name: 'Support, ACME', email: 'help@acme.com' }] };
+    await prepareSecureSend(msg, { mode: 'pgp', sign: true, encrypt: false }, {
+      fetchBlob: async () => new Uint8Array([1]),
+      upload: async (bytes) => { uploaded = bytes; return 'x'; },
+    });
+    const text = new TextDecoder().decode(uploaded!);
+    expect(text).toMatch(/^Reply-To: "Support, ACME" <help@acme.com>$/m);
+  }, 60_000);
+
   it('stops when a recipient has no key, or when the signing key is locked', async () => {
     await importPgpKey(me.privateKey, me.passphrase);
     const deps = { fetchBlob: async () => new Uint8Array(), upload: async () => 'x' };
diff --git a/src/lib/crypto/compose-secure.ts b/src/lib/crypto/compose-secure.ts
index 053f7b30..5135b5e9 100644
--- a/src/lib/crypto/compose-secure.ts
+++ b/src/lib/crypto/compose-secure.ts
@@ -8,7 +8,7 @@ import type { OutgoingEmail } from '../../api/email';
 import type { ContactCard } from '../../api/types';
 import { useCryptoKeysStore, keysForEmail, isRecordCurrent, type CryptoKeyRecord, type PgpKeyRecord, type SmimeCertRecord } from '../../stores/crypto-keys-store';
 import { messagePriorityHeaders } from '../message-priority';
-import { base64Decode, formatHeader, encodeHeaderWords, type MimeAttachment, type MimeMessageInput } from './mime';
+import { base64Decode, formatHeader, formatAddress, type MimeAttachment, type MimeMessageInput } from './mime';
 import { buildSecureMessage, type SecureMode } from './secure-send';
 import { ownKeyFor, unlockedPgpKey, unlockedSmimeKey } from './keyring';
 
@@ -100,7 +100,7 @@ export function secureReadiness(choice: SecureChoice, fromEmail: string, recipie
 function extraHeaders(outgoing: OutgoingEmail): string[] {
   const lines: string[] = [];
   if (outgoing.replyTo?.length) {
-    lines.push(formatHeader('Reply-To', outgoing.replyTo.map((a) => (a.name ? `${encodeHeaderWords(a.name)} <${a.email}>` : a.email)).join(', ')));
+    lines.push(formatHeader('Reply-To', outgoing.replyTo.map(formatAddress).join(', ')));
   }
   for (const [prop, value] of Object.entries(messagePriorityHeaders(outgoing.priority))) {
     const name = /^header:([^:]+):/.exec(prop)?.[1];
diff --git a/src/lib/crypto/mime.ts b/src/lib/crypto/mime.ts
index e6df1a96..71f8da03 100644
--- a/src/lib/crypto/mime.ts
+++ b/src/lib/crypto/mime.ts
@@ -90,7 +90,8 @@ function unfold(value: string | undefined): string {
   return String(value ?? '').replace(/[\r\n\t]+/g, ' ').replace(/ {2,}/g, ' ').trim();
 }
 
-function formatAddress(addr: MimeAddress): string {
+/** One address for a header: the name quoted (or RFC 2047-encoded) so commas in it stay inside. */
+export function formatAddress(addr: MimeAddress): string {
   if (addr.name) {
     const name = unfold(addr.name);
     // Raw 8-bit in a header is rejected by some MTAs; encode it.