diff --git a/stores/__tests__/auth-store-switch-guard.test.ts b/stores/__tests__/auth-store-switch-guard.test.ts
new file mode 100644
index 00000000..2bef56f0
--- /dev/null
+++ b/stores/__tests__/auth-store-switch-guard.test.ts
@@ -0,0 +1,52 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { connectedAccountId } from '../auth-store';
+import { useIdentityStore } from '../identity-store';
+import type { Identity } from '@/lib/jmap/types';
+
+// Covers the make-or-break bit of the account-switch identity guard: the
+// connected session's accountId must be derived with the SAME canonicalisation
+// as login (primary-identity email for OAuth, where the JMAP session username
+// is often a preferred_username claim, not the address). Comparing the raw
+// JMAP username would both false-positive on OAuth and miss real desyncs.
+
+const SERVER = 'https://mail.example.com';
+
+const fakeClient = (jmapUsername: string, identities: Identity[] | Error) =>
+  ({
+    getUsername: () => jmapUsername,
+    getIdentities: async () => {
+      if (identities instanceof Error) throw identities;
+      return identities;
+    },
+  }) as never;
+
+const id = (over: Partial<Identity> = {}): Identity => ({
+  id: 'id-1',
+  name: 'Real User',
+  email: 'real@example.com',
+  mayDelete: true,
+  ...over,
+});
+
+describe('connectedAccountId (account-switch guard)', () => {
+  beforeEach(() => {
+    useIdentityStore.setState({ identities: [], preferredPrimaryId: null } as never);
+  });
+
+  it('derives from the primary-identity EMAIL, not the JMAP session username', async () => {
+    // OAuth: JMAP username is a preferred_username claim, the real address lives
+    // on the identity. The id must be built from the email.
+    const result = await connectedAccountId(fakeClient('preferred_user', [id()]), SERVER);
+    expect(result).toBe('real@example.com@mail.example.com');
+  });
+
+  it('falls back to the JMAP username when identities cannot be fetched', async () => {
+    const result = await connectedAccountId(fakeClient('basic@example.com', new Error('no idents')), SERVER);
+    expect(result).toBe('basic@example.com@mail.example.com');
+  });
+
+  it('returns null when the session identity cannot be determined at all', async () => {
+    const broken = { getUsername: () => { throw new Error('disconnected'); } } as never;
+    expect(await connectedAccountId(broken, SERVER)).toBeNull();
+  });
+});
diff --git a/stores/auth-store.ts b/stores/auth-store.ts
index c117dffc..f351f788 100644
--- a/stores/auth-store.ts
+++ b/stores/auth-store.ts
@@ -415,6 +415,29 @@ function scheduleRefresh(expiresIn: number, refreshFn: () => Promise<string | nu
   }
 }
 
+/**
+ * Derive the accountId a *connected* client actually belongs to, using the
+ * same canonicalisation as login (primary-identity email for OAuth, else the
+ * JMAP session username). Lets a caller detect a slot->token mapping that
+ * resolves to the wrong account before it surfaces as the wrong mailbox.
+ * Returns null when it can't determine the identity (treated as "don't block").
+ */
+export async function connectedAccountId(client: JMAPClient, serverUrl: string): Promise<string | null> {
+  try {
+    const jmapUsername = client.getUsername();
+    let canonical = jmapUsername;
+    try {
+      const { primaryIdentity } = loadIdentities(await client.getIdentities(), jmapUsername);
+      canonical = primaryIdentity?.email || jmapUsername;
+    } catch {
+      /* identities unavailable — fall back to the JMAP session username */
+    }
+    return generateAccountId(canonical, serverUrl);
+  } catch {
+    return null;
+  }
+}
+
 function clearRefreshTimer(accountId?: string): void {
   if (accountId) {
     const timer = refreshTimers.get(accountId);
@@ -1363,6 +1386,26 @@ export const useAuthStore = create<AuthState>()(
           return;
         }
 
+        // GUARD: verify the connected session actually belongs to the target
+        // account before we bind it. A corrupted slot->token mapping (e.g.
+        // persisted client state left over from an older build, or any future
+        // slot desync) can hand back a *different* account's token; the
+        // connection then succeeds and we would silently show the wrong
+        // mailbox. On mismatch, drop the poisoned cookies for this slot and
+        // force a clean re-auth instead of surfacing someone else's mail.
+        const connectedId = await connectedAccountId(targetClient, targetAccount.serverUrl);
+        if (connectedId && connectedId !== accountId) {
+          debug.error(`switchAccount: slot ${targetAccount.cookieSlot} for ${accountId} resolved to ${connectedId} — forcing re-auth`);
+          clients.delete(accountId);
+          try { targetClient.disconnect(); } catch { /* noop */ }
+          apiFetch(`/api/auth/token?slot=${targetAccount.cookieSlot}`, { method: 'DELETE' }).catch(() => {});
+          apiFetch(`/api/auth/session?slot=${targetAccount.cookieSlot}`, { method: 'DELETE' }).catch(() => {});
+          accountStore.updateAccount(accountId, { isConnected: false, hasError: true, errorMessage: 'session_mismatch' });
+          set({ isLoading: false, error: 'connection_failed', activeAccountId: state.activeAccountId });
+          replaceWindowLocation(getLocaleLoginPath());
+          return;
+        }
+
         // Restore cached state or fetch fresh
         const restored = restoreAccount(accountId);
         accountStore.setActiveAccount(accountId);