diff --git a/stores/__tests__/email-store-unified-actions.test.ts b/stores/__tests__/email-store-unified-actions.test.ts
index 5256ee0e..de633ed3 100644
--- a/stores/__tests__/email-store-unified-actions.test.ts
+++ b/stores/__tests__/email-store-unified-actions.test.ts
@@ -68,6 +68,7 @@ describe('unified-view single-email action routing (#281)', () => {
     // account-a is the active login; account-b is a second direct login; the
     // active login (account-a) also delegates access to the shared owner 'owner-x'.
     useAuthStore.setState({
+      activeAccountId: 'account-a',
       getClientForAccount: (id: string) =>
         (id === 'account-b' ? accountBClient : id === 'account-a' ? activeClient : undefined) as never,
     } as never);
@@ -124,6 +125,27 @@ describe('unified-view single-email action routing (#281)', () => {
     expect(activeClient.moveEmail).not.toHaveBeenCalled();
   });
 
+  it('updates the unread counter on the email’s own account, not the active one (id collision)', async () => {
+    // Real per-account JMAP ids can collide; here both inboxes use the same id.
+    useEmailStore.setState({
+      mailboxes: [makeMailbox({ id: 'inbox', role: 'inbox', unreadEmails: 5 })],
+      accountMailboxes: {
+        'account-a': [makeMailbox({ id: 'inbox', role: 'inbox', unreadEmails: 5 })],
+        'account-b': [makeMailbox({ id: 'inbox', role: 'inbox', unreadEmails: 3 })],
+      },
+      emails: [
+        makeEmail({ id: 'b1', sourceClientAccountId: 'account-b', sourceAccountId: 'account-b', keywords: {}, mailboxIds: { inbox: true } }),
+      ],
+    });
+
+    await useEmailStore.getState().markAsRead(activeClient, 'b1', true);
+
+    const s = useEmailStore.getState();
+    expect(s.accountMailboxes['account-b'][0].unreadEmails).toBe(2); // account-b decremented
+    expect(s.mailboxes[0].unreadEmails).toBe(5);                     // active account untouched
+    expect(s.accountMailboxes['account-a'][0].unreadEmails).toBe(5); // active list untouched
+  });
+
   it('routes a shared/group email through the delegating login client + owner accountId', async () => {
     await useEmailStore.getState().markAsRead(activeClient, 'email-shared', true);
     // Reached via account-a's client (the active one), targeting the owner account.
diff --git a/stores/email-store.ts b/stores/email-store.ts
index 6e4a764a..b50a3556 100644
--- a/stores/email-store.ts
+++ b/stores/email-store.ts
@@ -522,6 +522,34 @@ function emailInMailbox(
   return false;
 }
 
+// Apply a per-mailbox counter adjustment to the mailbox list that actually holds
+// the email's folders, and return the matching `set()` partial.
+//
+// In aggregate views an email may belong to another logged-in account. The
+// sidebar shows the active account's folders from `mailboxes` (its
+// getAllMailboxes, incl. its delegated shared folders) and every *other*
+// account's folders from `accountMailboxes[<AccountEntry.id>]`. Updating the
+// wrong list silently corrupts counters - and because JMAP mailbox ids can
+// collide across accounts, blindly matching `mailboxes` would even decrement the
+// *active* account's folder for a different account's email. So route by the
+// email's source login: a different account → its `accountMailboxes` entry,
+// otherwise (active account, its shared folders, or a non-aggregate view) →
+// `mailboxes`. (#281)
+function applyMailboxCounterUpdate(
+  state: { mailboxes: Mailbox[]; accountMailboxes: Record<string, Mailbox[]> },
+  email: { sourceClientAccountId?: string },
+  adjust: (mb: Mailbox) => Mailbox,
+): { mailboxes?: Mailbox[]; accountMailboxes?: Record<string, Mailbox[]> } {
+  const srcClient = email.sourceClientAccountId;
+  const activeId = useAuthStore.getState().activeAccountId;
+  if (srcClient && srcClient !== activeId) {
+    const list = state.accountMailboxes[srcClient];
+    if (!list) return {};
+    return { accountMailboxes: { ...state.accountMailboxes, [srcClient]: list.map(adjust) } };
+  }
+  return { mailboxes: state.mailboxes.map(adjust) };
+}
+
 // Find the trash mailbox for a given account scope. Prefers JMAP role, but
 // falls back to name matching ("trash" / "deleted") so users with custom or
 // pre-existing folders (e.g. "Deleted Items") aren't silently destroyed.
@@ -1327,21 +1355,18 @@ export const useEmailStore = create<EmailStore>((set, get) => ({
           return { processingReadStatus: newProcessingSet }; // State unchanged, skip counter update
         }
 
-        const updatedMailboxes = state.mailboxes.map(mailbox => {
-          // Check if this email belongs to this mailbox. Shared mailboxes are
-          // stored under a namespaced id but the email's mailboxIds are keyed by
-          // the owner-side JMAP id (originalId), so match on originalId first.
-          if (emailInMailbox(emailInState, mailbox)) {
-            // Adjust unread counter: -1 if marking as read, +1 if marking as unread
-            const delta = read ? -1 : 1;
-            return {
-              ...mailbox,
-              unreadEmails: Math.max(0, mailbox.unreadEmails + delta),
-              unreadThreads: Math.max(0, mailbox.unreadThreads + delta)
-            };
-          }
-          return mailbox;
-        });
+        // Adjust the unread counter on the folder(s) holding this email, in the
+        // email's *own* account's mailbox list (#281). -1 marking read, +1 unread.
+        const delta = read ? -1 : 1;
+        const mailboxPatch = applyMailboxCounterUpdate(state, emailInState, (mailbox) =>
+          emailInMailbox(emailInState, mailbox)
+            ? {
+                ...mailbox,
+                unreadEmails: Math.max(0, mailbox.unreadEmails + delta),
+                unreadThreads: Math.max(0, mailbox.unreadThreads + delta),
+              }
+            : mailbox,
+        );
 
         return {
           emails: state.emails.map(e =>
@@ -1350,7 +1375,7 @@ export const useEmailStore = create<EmailStore>((set, get) => ({
           selectedEmail: state.selectedEmail?.id === emailId
             ? { ...state.selectedEmail, keywords: { ...state.selectedEmail.keywords, $seen: read } }
             : state.selectedEmail,
-          mailboxes: updatedMailboxes,
+          ...mailboxPatch,
           processingReadStatus: newProcessingSet,
           // Also update threadEmailsCache so expanded dropdowns reflect the change
           threadEmailsCache: (() => {