diff --git a/components/layout/__tests__/sidebar-unified-section.test.tsx b/components/layout/__tests__/sidebar-unified-section.test.tsx
new file mode 100644
index 000000000..1501f126b
--- /dev/null
+++ b/components/layout/__tests__/sidebar-unified-section.test.tsx
@@ -0,0 +1,72 @@
+import { render, screen } from '@testing-library/react';
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import type { Mailbox } from '@/lib/jmap/types';
+
+// #843: the "Unified Mailbox" header used to appear as soon as two accounts
+// were connected, but mail-app only populates the section when the admin gate
+// and the user's cross-account toggle are both on (`crossAccountActive`), or a
+// merged group inbox / cross view exists. An empty header is not shown.
+
+vi.mock('@/components/plugins/plugin-slot', () => ({ PluginSlot: () => null }));
+vi.mock('@/components/tour/tour-provider', () => ({
+  useTour: () => ({ startTour: vi.fn(), resetTourCompletion: vi.fn(), activeTour: null, endTour: vi.fn(), registerStep: vi.fn() }),
+}));
+vi.mock('@/contexts/drag-drop-context', () => ({
+  useDragDropContext: () => ({ isDragging: false }),
+}));
+vi.mock('@/hooks/use-mailbox-drop', () => ({
+  useMailboxDrop: () => ({ dropHandlers: {}, isValidDropTarget: false, isInvalidDropTarget: false }),
+}));
+vi.mock('@/hooks/use-tag-drop', () => ({
+  useTagDrop: () => ({ dropHandlers: {}, isValidDropTarget: false }),
+}));
+vi.mock('../account-switcher', () => ({ AccountSwitcher: () => null }));
+
+const { Sidebar } = await import('../sidebar');
+const { useAccountStore } = await import('@/stores/account-store');
+const { useSettingsStore } = await import('@/stores/settings-store');
+
+const mailboxes: Mailbox[] = [
+  { id: 'inbox', name: 'Inbox', role: 'inbox', parentId: null, totalEmails: 0, unreadEmails: 0, myRights: { mayAddItems: true } },
+] as unknown as Mailbox[];
+
+function account(id: string) {
+  return {
+    id,
+    label: `${id}@example.com`,
+    email: `${id}@example.com`,
+    username: `${id}@example.com`,
+    cookieSlot: id === 'a' ? 0 : 1,
+    isConnected: true,
+  };
+}
+
+describe('Sidebar unified mailbox section (#843)', () => {
+  beforeEach(() => {
+    useAccountStore.setState({ accounts: [account('a'), account('b')] } as never);
+    useSettingsStore.setState({
+      enableUnifiedMailbox: true,
+      includeGroupInUnified: false,
+      hideAccountSwitcher: true,
+    } as never);
+  });
+
+  it('hides the header with two accounts when cross-account is not active', () => {
+    render(<Sidebar mailboxes={mailboxes} selectedMailbox="inbox" crossAccountActive={false} />);
+
+    expect(screen.queryByText('unified_mailbox')).toBeNull();
+    expect(screen.queryByText('all_accounts')).toBeNull();
+  });
+
+  it('shows the header once cross-account is active', () => {
+    render(<Sidebar mailboxes={mailboxes} selectedMailbox="inbox" crossAccountActive />);
+
+    expect(screen.getByText('all_accounts')).toBeInTheDocument();
+  });
+
+  it('shows the header for a cross view even without cross-account', () => {
+    render(<Sidebar mailboxes={mailboxes} selectedMailbox="inbox" crossAccountActive={false} showCrossUnread />);
+
+    expect(screen.getByText('unified_mailbox')).toBeInTheDocument();
+  });
+});
diff --git a/components/layout/sidebar.tsx b/components/layout/sidebar.tsx
index 92668a88f..297442b60 100644
--- a/components/layout/sidebar.tsx
+++ b/components/layout/sidebar.tsx
@@ -904,9 +904,14 @@ export function Sidebar({
   // single account we still surface unified when the user has opted into
   // merging group/shared inboxes — otherwise the counts would just duplicate
   // the one inbox.
+  // Only show the section when mail-app will actually populate it: cross-account
+  // needs the admin gate + user toggle (`crossAccountActive`, which already
+  // implies 2+ connected accounts), otherwise a merged group inbox or one of
+  // the cross views must exist. A bare "2+ accounts" left an empty header. (#843)
+  const anyCrossViewEnabled = showCrossUnread || showCrossStarred || showCrossAll;
   const showUnified =
     (multiAccountMode || enableUnifiedMailbox) &&
-    (connectedAccounts.length > 1 || (includeGroupInUnified && hasGroupInboxes));
+    (crossAccountActive || (includeGroupInUnified && hasGroupInboxes) || anyCrossViewEnabled);
   const { unifiedCounts } = useEmailStore();
   const t = useTranslations('sidebar');