diff --git a/lib/__tests__/auth-routes-credential-check.test.ts b/lib/__tests__/auth-routes-credential-check.test.ts
index 222f549e9..3c49cbe32 100644
--- a/lib/__tests__/auth-routes-credential-check.test.ts
+++ b/lib/__tests__/auth-routes-credential-check.test.ts
@@ -303,12 +303,15 @@ describe('POST /api/auth/stalwart-context (GHSA-wxcm-j4jc-9fxq)', () => {
     expect(setStalwartAuthContext).toHaveBeenCalledWith(0, expect.objectContaining({ username: 'linus@example.org' }));
   });
 
-  it('accepts the bare local part of the session username (Stalwart canonicalizes it)', async () => {
+  it('refuses an address on a domain the bare session username does not name', async () => {
+    // `lrath` holds no address at corp.example; lrath@corp.example may be
+    // another account's login, with its own synced settings.
     const { status } = await postContext({
       serverUrl: SERVER, username: 'lrath@corp.example', authHeader: 'Bearer tok-linus', slot: 0,
     });
-    expect(status).toBe(200);
-    expect(identityGetCalls).toBe(0);
+    expect(status).toBe(403);
+    expect(identityGetCalls).toBe(1);
+    expect(setStalwartAuthContext).not.toHaveBeenCalled();
   });
 
   it('still mints the context for a Basic credential the server accepts', async () => {
diff --git a/lib/__tests__/verify-jmap-identity.test.ts b/lib/__tests__/verify-jmap-identity.test.ts
index 0c2e3d6c4..82ab4616a 100644
--- a/lib/__tests__/verify-jmap-identity.test.ts
+++ b/lib/__tests__/verify-jmap-identity.test.ts
@@ -47,10 +47,15 @@ describe('usernameMatchesSession', () => {
     expect(usernameMatchesSession('UserA@Example.org', 'usera@example.org')).toBe(true);
   });
 
-  it('matches a bare local part against the canonical e-mail login on either side', async () => {
+  it('matches a bare local part against the canonical e-mail login', async () => {
     const { usernameMatchesSession } = await load();
     expect(usernameMatchesSession('usera', 'usera@example.org')).toBe(true);
-    expect(usernameMatchesSession('linus@example.org', 'linus')).toBe(true);
+  });
+
+  it('does not let a bare login name vouch for an address on any domain', async () => {
+    // `linus` says nothing about which domain is his; his identities do.
+    const { usernameMatchesSession } = await load();
+    expect(usernameMatchesSession('linus@example.org', 'linus')).toBe(false);
   });
 
   it('rejects different users, different domains and missing session usernames', async () => {
@@ -206,6 +211,24 @@ describe('resolveJmapIdentity account name', () => {
       .resolves.toEqual({ serverUrl: 'https://mail.internal', accountName: 'John@B.example' });
   });
 
+  it('does not key a bare login on an address of another domain', async () => {
+    // `info` (info@a.example) claiming info@b.example, another account's
+    // login, would read and write that account's synced settings.
+    const session = {
+      apiUrl: 'https://mail.internal/jmap/',
+      username: 'info',
+      primaryAccounts: { 'urn:ietf:params:jmap:mail': 'c' },
+      accounts: {},
+    };
+    const identities = { methodResponses: [['Identity/get', { accountId: 'c', list: [{ id: 'i0', email: 'info@a.example' }] }, '0']] };
+    fetchSpy.mockImplementation(async (url: RequestInfo | URL) => json(String(url).endsWith('/.well-known/jmap') ? session : identities));
+    const { resolveJmapIdentity } = await load();
+    await expect(resolveJmapIdentity('https://mail.internal', 'Bearer tok', 'info@b.example', { trusted: true }))
+      .rejects.toMatchObject({ status: 403 });
+    await expect(resolveJmapIdentity('https://mail.internal', 'Bearer tok', 'info@a.example', { trusted: true }))
+      .resolves.toEqual({ serverUrl: 'https://mail.internal', accountName: 'info@a.example' });
+  });
+
   it('keeps a Basic claim as it is: the server accepted exactly that login', async () => {
     fetchSpy.mockResolvedValueOnce(json({ apiUrl: 'https://mail.internal/jmap/', username: 'john@a.example', accounts: {} }));
     const { resolveJmapIdentity } = await load();
diff --git a/lib/auth/verify-jmap-auth.ts b/lib/auth/verify-jmap-auth.ts
index 5ddc0cc42..05f9fb86a 100644
--- a/lib/auth/verify-jmap-auth.ts
+++ b/lib/auth/verify-jmap-auth.ts
@@ -189,19 +189,18 @@ function usernamesEqual(a: string, b: string): boolean {
  * Does the claimed login name denote the server-reported one? Stalwart
  * canonicalizes the login it echoes in `Session.username` (`UserA@x` and the
  * bare local part `usera` both come back as `usera@x`), so besides a
- * case-insensitive match we accept a bare local part on either side.
+ * case-insensitive match we accept a bare local part as the claim. Not the
+ * other way round: a bare `Session.username` names no domain, so `info`
+ * would vouch for `info@` any domain, another account's login among them.
+ * Such a claim has to be one of the account's identities instead.
  */
 export function usernameMatchesSession(claimed: string, sessionUsername: string | undefined): boolean {
   if (!sessionUsername) return false;
   if (usernamesEqual(claimed, sessionUsername)) return true;
-  const claimedAt = claimed.indexOf('@');
   const sessionAt = sessionUsername.indexOf('@');
-  if (claimedAt < 0 && sessionAt > 0) {
+  if (!claimed.includes('@') && sessionAt > 0) {
     return usernamesEqual(claimed, sessionUsername.slice(0, sessionAt));
   }
-  if (sessionAt < 0 && claimedAt > 0) {
-    return usernamesEqual(claimed.slice(0, claimedAt), sessionUsername);
-  }
   return false;
 }