diff --git a/src/lib/__tests__/account-id-port.test.ts b/src/lib/__tests__/account-id-port.test.ts
index 558f8ce0..18a826a6 100644
--- a/src/lib/__tests__/account-id-port.test.ts
+++ b/src/lib/__tests__/account-id-port.test.ts
@@ -49,6 +49,45 @@ describe('generateAccountId', () => {
   });
 });
 
+describe('a new default-port account next to a kept host-only id', () => {
+  const legacy = { id: 'u@example.org@mail.example.org', username: 'u@example.org', serverUrl: 'https://mail.example.org:8120' };
+
+  it('gets an id of its own instead of the one the other server kept', () => {
+    setKnownAccounts([legacy]);
+    expect(generateAccountId('u@example.org', 'https://mail.example.org')).toBe('u@example.org@mail.example.org:443');
+    expect(generateAccountId('u@example.org', 'http://mail.example.org/jmap')).toBe('u@example.org@mail.example.org:80');
+    // The kept account itself still resolves to its old id.
+    expect(generateAccountId('u@example.org', 'https://mail.example.org:8120')).toBe(legacy.id);
+  });
+
+  it('keeps that id after the other account is gone', () => {
+    const own = { id: 'u@example.org@mail.example.org:443', username: 'u@example.org', serverUrl: 'https://mail.example.org' };
+    setKnownAccounts([legacy, own]);
+    expect(generateAccountId('u@example.org', 'https://mail.example.org')).toBe(own.id);
+    setKnownAccounts([own]);
+    expect(generateAccountId('u@example.org', 'https://mail.example.org')).toBe(own.id);
+  });
+
+  it('leaves an existing default-port account under its plain id', () => {
+    setKnownAccounts([{ id: 'u@example.org@mail.example.org', username: 'u@example.org', serverUrl: 'https://mail.example.org/' }]);
+    expect(generateAccountId('u@example.org', 'https://mail.example.org')).toBe('u@example.org@mail.example.org');
+  });
+
+  it('adds it to the registry as a second account', () => {
+    useAccountStore.setState({
+      accounts: [{ ...entry('u@example.org', legacy.serverUrl), id: legacy.id, avatarColor: '#000', isDefault: true }],
+      activeAccountId: legacy.id,
+      defaultAccountId: legacy.id,
+    });
+    const id = useAccountStore.getState().addAccount(entry('u@example.org', 'https://mail.example.org'));
+    expect(id).toBe('u@example.org@mail.example.org:443');
+    expect(useAccountStore.getState().accounts.map((x) => [x.id, x.serverUrl])).toEqual([
+      [legacy.id, legacy.serverUrl],
+      ['u@example.org@mail.example.org:443', 'https://mail.example.org'],
+    ]);
+  });
+});
+
 describe('the account registry', () => {
   it('adds the same address on another port of the same host as a second account', () => {
     const store = useAccountStore.getState();
diff --git a/src/lib/account-utils.ts b/src/lib/account-utils.ts
index 13903bfd..f60eb6e3 100644
--- a/src/lib/account-utils.ts
+++ b/src/lib/account-utils.ts
@@ -50,6 +50,11 @@ function hostOnlyAccountId(username: string, serverUrl: string): string {
   return `${username}@${host}`;
 }
 
+/** The default port of `serverUrl`'s scheme: 80 for http, 443 otherwise. */
+function defaultPortOf(serverUrl: string): string {
+  return /^http:/i.test(serverUrl.trim()) ? '80' : '443';
+}
+
 /** What the registry tells generateAccountId (structural: no AccountEntry import). */
 export interface KnownAccount {
   id: string;
@@ -78,13 +83,22 @@ export function setKnownAccounts(accounts: readonly KnownAccount[]): void {
  * outbox, push registration and settings: when the registry holds the
  * host-only id for the same server, that is the answer. A second server on
  * the same host then gets the id with its port, and the two stay apart.
- * Accounts on the default ports have the same id either way.
+ * Accounts on the default ports have the same id either way, with one
+ * exception: when the registry holds username@host for a server on another
+ * port of that host (kept from before ports counted), the default-port
+ * server gets username@host:443 (or :80 for http), so the two don't share
+ * credentials and caches. It keeps that id once it has it.
  */
 export function generateAccountId(username: string, serverUrl: string): string {
   const legacy = hostOnlyAccountId(username, serverUrl);
   const host = serverHostLabel(serverUrl);
   const id = `${username}@${host}`;
-  if (id === legacy) return id;
+  if (id === legacy) {
+    const explicit = `${id}:${defaultPortOf(serverUrl)}`;
+    if (knownAccounts.some((a) => a.id === explicit)) return explicit;
+    const taken = knownAccounts.some((a) => a.id === id && `${a.username}@${serverHostLabel(a.serverUrl)}` !== id);
+    return taken ? explicit : id;
+  }
   const kept = knownAccounts.find((a) => a.id === legacy
     && a.username === username
     && `${a.username}@${serverHostLabel(a.serverUrl)}` === id);