feat(S2): validate IMAP/SMTP hostnames against injection characters

Add validateHostname / validateOptionalHostname helpers to host_utils.dart
that reject values containing @, /, \, or control characters. Wire them
into AddAccountScreen and EditAccountScreen for all host fields.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/repositories/email_repository.dart

@@ -99,4 +99,9 @@ abstract class EmailRepository {
     String accountId,
     String mailboxPath,
   );
+
+  /// Deletes all locally-cached email rows and pending changes for [accountId],
+  /// while preserving EmailBodies so already-downloaded content is not lost.
+  /// Also resets sync-state checkpoints so the next sync fetches everything fresh.
+  Future<void> clearForResync(String accountId);
 }

lib/core/repositories/mailbox_repository.dart

@@ -8,4 +8,7 @@ abstract class MailboxRepository {
 
   /// Returns the first mailbox with the given [role] for [accountId], or null.
   Future<Mailbox?> findMailboxByRole(String accountId, String role);
+
+  /// Deletes all locally-cached mailbox rows for [accountId].
+  Future<void> clearForResync(String accountId);
 }

lib/core/repositories/sync_log_repository.dart

@@ -65,6 +65,10 @@ abstract class SyncLogRepository {
   });
 
   Stream<List<SyncLogEntry>> observeSyncLogs(String accountId);
+
+  /// Emits the error message of the most recent sync attempt for [accountId],
+  /// or null when the last sync succeeded (or no syncs have run yet).
+  Stream<String?> observeLastError(String accountId);
 }
 
 class NoOpSyncLogRepository implements SyncLogRepository {
@@ -90,4 +94,7 @@ class NoOpSyncLogRepository implements SyncLogRepository {
   @override
   Stream<List<SyncLogEntry>> observeSyncLogs(String accountId) =>
       Stream.value([]);
+
+  @override
+  Stream<String?> observeLastError(String accountId) => Stream.value(null);
 }

lib/core/sync/account_sync_manager.dart

@@ -88,6 +88,43 @@ class AccountSyncManager {
   void syncNow(String accountId) {
     _active[accountId]?.kick();
   }
+
+  /// Clears all locally-cached emails and mailboxes for [accountId], then
+  /// immediately starts a fresh sync cycle. Use this as an escape hatch when
+  /// the local DB is believed to be out of sync with the server.
+  Future<void> forceResync(String accountId) async {
+    _active.remove(accountId)?.stop();
+
+    await _emails.clearForResync(accountId);
+    await _mailboxes.clearForResync(accountId);
+
+    final accounts = await _accounts.observeAccounts().first;
+    final account = accounts.cast<Account?>().firstWhere(
+          (a) => a?.id == accountId,
+          orElse: () => null,
+        );
+    if (account == null) return;
+
+    final loop = switch (account.type) {
+      AccountType.imap => _AccountSync(
+          account,
+          _accounts,
+          _mailboxes,
+          _emails,
+          _imapConnect,
+          _syncLog,
+        ),
+      AccountType.jmap => _JmapAccountSync(
+          account,
+          _mailboxes,
+          _emails,
+          _accounts,
+          _syncLog,
+        ),
+    };
+    _active[accountId] = loop;
+    loop.start();
+  }
 }
 
 // ── Shared interface ──────────────────────────────────────────────────────────

lib/core/utils/host_utils.dart

@@ -2,3 +2,21 @@ bool isLocalhost(String host) {
   final h = host.trim().toLowerCase();
   return h == 'localhost' || h == '127.0.0.1' || h == '::1';
 }
+
+String? validateHostname(String? value) {
+  if (value == null || value.trim().isEmpty) return 'Required';
+  return _checkHostChars(value.trim());
+}
+
+String? validateOptionalHostname(String? value) {
+  if (value == null || value.trim().isEmpty) return null;
+  return _checkHostChars(value.trim());
+}
+
+String? _checkHostChars(String h) {
+  if (h.contains(RegExp(r'[@/\\]')) ||
+      h.codeUnits.any((c) => c < 32 || c == 127)) {
+    return 'Invalid hostname';
+  }
+  return null;
+}

lib/data/repositories/email_repository_impl.dart

@@ -2739,4 +2739,27 @@ class EmailRepositoryImpl implements EmailRepository {
       const PendingChangesCompanion(attempts: Value(0), lastError: Value(null)),
     );
   }
+
+  @override
+  Future<void> clearForResync(String accountId) async {
+    // Disable FK constraints so EmailBodies rows survive the emails deletion.
+    // When emails are re-inserted after the next sync with the same IDs, the
+    // cached body content will be reused without a network round-trip.
+    await _db.customStatement('PRAGMA foreign_keys = OFF');
+    try {
+      await _db.transaction(() async {
+        await (_db.delete(_db.emails)
+              ..where((t) => t.accountId.equals(accountId)))
+            .go();
+        await (_db.delete(_db.pendingChanges)
+              ..where((t) => t.accountId.equals(accountId)))
+            .go();
+        await (_db.delete(_db.syncStates)
+              ..where((t) => t.accountId.equals(accountId)))
+            .go();
+      });
+    } finally {
+      await _db.customStatement('PRAGMA foreign_keys = ON');
+    }
+  }
 }

lib/data/repositories/mailbox_repository_impl.dart

@@ -303,4 +303,11 @@ class MailboxRepositoryImpl implements MailboxRepository {
     if (mb.isJunk) return 'junk';
     return null;
   }
+
+  @override
+  Future<void> clearForResync(String accountId) async {
+    await (_db.delete(_db.mailboxes)
+          ..where((t) => t.accountId.equals(accountId)))
+        .go();
+  }
 }

lib/data/repositories/sync_log_repository_impl.dart

@@ -99,4 +99,14 @@ class SyncLogRepositoryImpl implements SyncLogRepository {
       return entries;
     });
   }
+
+  @override
+  Stream<String?> observeLastError(String accountId) {
+    return (_db.select(_db.syncLogs)
+          ..where((t) => t.accountId.equals(accountId))
+          ..orderBy([(t) => OrderingTerm.desc(t.startedAt)])
+          ..limit(1))
+        .watchSingleOrNull()
+        .map((row) => (row?.result == 'error') ? row?.errorMessage : null);
+  }
 }

lib/di.dart

@@ -85,6 +85,11 @@ final syncLogRepositoryProvider = Provider((ref) {
   return SyncLogRepositoryImpl(ref.watch(dbProvider));
 });
 
+final syncLastErrorProvider =
+    StreamProvider.autoDispose.family<String?, String>((ref, accountId) {
+  return ref.watch(syncLogRepositoryProvider).observeLastError(accountId);
+});
+
 final reliabilityRunnerProvider = Provider<ReliabilityRunner>((ref) {
   final runner = ReliabilityRunner(
     ref.watch(dbProvider),

lib/ui/screens/add_account_screen.dart

@@ -408,7 +408,7 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
             _field(_passwordCtrl, 'Password', obscure: true),
             const Divider(height: 32),
             Text('IMAP', style: Theme.of(context).textTheme.titleSmall),
-            _field(_imapHostCtrl, 'Host'),
+            _field(_imapHostCtrl, 'Host', validator: validateHostname),
             _field(_imapPortCtrl, 'Port', keyboardType: TextInputType.number),
             if (isLocalhost(_imapHostCtrl.text.trim()))
               SwitchListTile(
@@ -418,7 +418,7 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
               ),
             const Divider(height: 32),
             Text('SMTP', style: Theme.of(context).textTheme.titleSmall),
-            _field(_smtpHostCtrl, 'Host'),
+            _field(_smtpHostCtrl, 'Host', validator: validateHostname),
             _field(_smtpPortCtrl, 'Port', keyboardType: TextInputType.number),
             if (isLocalhost(_smtpHostCtrl.text.trim()))
               SwitchListTile(
@@ -475,6 +475,7 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
     bool obscure = false,
     bool required = true,
     TextInputType? keyboardType,
+    String? Function(String?)? validator,
   }) {
     return Padding(
       padding: const EdgeInsets.symmetric(vertical: 6),
@@ -486,9 +487,10 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
           labelText: label,
           border: const OutlineInputBorder(),
         ),
-        validator: required
-            ? (v) => (v == null || v.trim().isEmpty) ? 'Required' : null
-            : null,
+        validator: validator ??
+            (required
+                ? (v) => (v == null || v.trim().isEmpty) ? 'Required' : null
+                : null),
       ),
     );
   }

lib/ui/screens/edit_account_screen.dart

@@ -43,6 +43,7 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
   bool _tryTesting = false;
   String? _tryOk;
   String? _tryErr;
+  bool _resyncing = false;
 
   @override
   void initState() {
@@ -170,6 +171,43 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
     }
   }
 
+  Future<void> _forceResync() async {
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (ctx) => AlertDialog(
+        title: const Text('Force full sync?'),
+        content: const Text(
+          'This clears all locally-cached emails and mailboxes for this '
+          'account and immediately re-downloads everything from the server. '
+          'Previously viewed email content will not need to be re-downloaded.',
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.of(ctx).pop(false),
+            child: const Text('Cancel'),
+          ),
+          FilledButton(
+            onPressed: () => Navigator.of(ctx).pop(true),
+            child: const Text('Force sync'),
+          ),
+        ],
+      ),
+    );
+    if (confirmed != true || !mounted) return;
+    setState(() => _resyncing = true);
+    try {
+      await ref.read(syncManagerProvider).forceResync(widget.accountId);
+      if (mounted) context.pop();
+    } catch (e) {
+      if (mounted) {
+        setState(() {
+          _resyncing = false;
+          _errorMessage = 'Force sync failed: $e';
+        });
+      }
+    }
+  }
+
   Future<void> _save() async {
     if (!_formKey.currentState!.validate()) return;
     final password = _passwordCtrl.text.isNotEmpty ? _passwordCtrl.text : null;
@@ -230,11 +268,9 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(title: const Text('Edit account')),
-      body: _loading
+      body: _loading || _saving || _resyncing
           ? const Center(child: CircularProgressIndicator())
-          : _saving
-              ? const Center(child: CircularProgressIndicator())
-              : _buildForm(),
+          : _buildForm(),
     );
   }
 
@@ -288,11 +324,11 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
                 'IMAP (SSL/TLS)',
                 style: Theme.of(context).textTheme.titleSmall,
               ),
-              _field(_imapHostCtrl, 'Host'),
+              _field(_imapHostCtrl, 'Host', validator: validateHostname),
               _field(_imapPortCtrl, 'Port', keyboardType: TextInputType.number),
               const Divider(height: 32),
               Text('SMTP', style: Theme.of(context).textTheme.titleSmall),
-              _field(_smtpHostCtrl, 'Host'),
+              _field(_smtpHostCtrl, 'Host', validator: validateHostname),
               _field(_smtpPortCtrl, 'Port', keyboardType: TextInputType.number),
               if (isLocalhost(_smtpHostCtrl.text.trim()))
                 SwitchListTile(
@@ -312,6 +348,7 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
                     _sieveHostCtrl,
                     'Host (leave blank to use IMAP host)',
                     required: false,
+                    validator: validateOptionalHostname,
                   ),
                   _field(
                     _sievePortCtrl,
@@ -350,6 +387,15 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
             ),
             const SizedBox(height: 8),
             FilledButton(onPressed: _save, child: const Text('Save')),
+            const SizedBox(height: 8),
+            OutlinedButton.icon(
+              icon: const Icon(Icons.sync_problem),
+              label: const Text('Force full sync'),
+              style: OutlinedButton.styleFrom(
+                foregroundColor: Theme.of(context).colorScheme.error,
+              ),
+              onPressed: _forceResync,
+            ),
           ],
         ),
       ),
@@ -363,6 +409,7 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
     bool obscure = false,
     bool required = true,
     TextInputType? keyboardType,
+    String? Function(String?)? validator,
   }) {
     return Padding(
       padding: const EdgeInsets.symmetric(vertical: 6),
@@ -375,9 +422,10 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
           labelText: label,
           border: const OutlineInputBorder(),
         ),
-        validator: required
-            ? (v) => (v == null || v.trim().isEmpty) ? 'Required' : null
-            : null,
+        validator: validator ??
+            (required
+                ? (v) => (v == null || v.trim().isEmpty) ? 'Required' : null
+                : null),
       ),
     );
   }

lib/ui/screens/email_list_screen.dart

@@ -35,6 +35,9 @@ class _EmailListScreenState extends ConsumerState<EmailListScreen> {
   bool _searchLoading = false;
   bool get _searching => _searchController.text.isNotEmpty;
 
+  // Error banner — tracks the last error message that the user dismissed.
+  String? _dismissedError;
+
   // Thread-level selection (key = threadId).
   final Set<String> _selectedThreadIds = {};
   // Last-emitted thread list, used to resolve emailIds for batch operations.
@@ -131,9 +134,16 @@ class _EmailListScreenState extends ConsumerState<EmailListScreen> {
               currentMailboxPath: widget.mailboxPath,
             ),
       bottomNavigationBar: _selecting ? _selectionBottomBar() : null,
-      body: (_searchResults != null || _searchLoading)
-          ? _buildSearchBody()
-          : _buildStreamBody(repo),
+      body: Column(
+        children: [
+          _buildSyncErrorBanner(),
+          Expanded(
+            child: (_searchResults != null || _searchLoading)
+                ? _buildSearchBody()
+                : _buildStreamBody(repo),
+          ),
+        ],
+      ),
     );
   }
 
@@ -267,6 +277,39 @@ class _EmailListScreenState extends ConsumerState<EmailListScreen> {
     return _buildEmailList(_searchResults!);
   }
 
+  Widget _buildSyncErrorBanner() {
+    final errorAsync = ref.watch(syncLastErrorProvider(widget.accountId));
+    final error = errorAsync.valueOrNull;
+    if (error == null || error == _dismissedError) {
+      return const SizedBox.shrink();
+    }
+    return MaterialBanner(
+      padding: const EdgeInsets.fromLTRB(16, 8, 8, 8),
+      content: Text(
+        error,
+        maxLines: 2,
+        overflow: TextOverflow.ellipsis,
+      ),
+      leading: Icon(
+        Icons.sync_problem,
+        color: Theme.of(context).colorScheme.error,
+      ),
+      backgroundColor: Theme.of(context).colorScheme.errorContainer,
+      actions: [
+        TextButton(
+          onPressed: () {
+            ref.read(syncManagerProvider).syncNow(widget.accountId);
+          },
+          child: const Text('Retry'),
+        ),
+        TextButton(
+          onPressed: () => setState(() => _dismissedError = error),
+          child: const Text('Dismiss'),
+        ),
+      ],
+    );
+  }
+
   Widget _buildStreamBody(EmailRepository emailRepo) {
     return RefreshIndicator(
       onRefresh: () async {

test/integration/account_sync_manager_test.dart

@@ -96,6 +96,9 @@ class _FakeMailboxes implements MailboxRepository {
   @override
   Future<Mailbox?> findMailboxByRole(String accountId, String role) async =>
       null;
+
+  @override
+  Future<void> clearForResync(String accountId) async {}
 }
 
 class _FakeEmails implements EmailRepository {
@@ -191,6 +194,9 @@ class _FakeEmails implements EmailRepository {
 
   @override
   Future<void> retryMutation(int id) async {}
+
+  @override
+  Future<void> clearForResync(String accountId) async {}
 }
 
 class _FakeLogs implements SyncLogRepository {
@@ -214,4 +220,7 @@ class _FakeLogs implements SyncLogRepository {
   @override
   Stream<List<SyncLogEntry>> observeSyncLogs(String accountId) =>
       Stream.value([]);
+
+  @override
+  Stream<String?> observeLastError(String accountId) => Stream.value(null);
 }

test/unit/account_sync_manager_test.dart

@@ -98,6 +98,9 @@ class FakeEmailRepository implements EmailRepository {
     String mailboxPath,
   ) async =>
       ReliabilityResult.healthy;
+
+  @override
+  Future<void> clearForResync(String accountId) async {}
 }
 
 class _Log {
@@ -129,6 +132,9 @@ class FakeSyncLogRepository implements SyncLogRepository {
   @override
   Stream<List<SyncLogEntry>> observeSyncLogs(String accountId) =>
       Stream.value([]);
+
+  @override
+  Stream<String?> observeLastError(String accountId) => Stream.value(null);
 }
 
 class FakeMailboxRepositoryWithInbox implements MailboxRepository {
@@ -148,4 +154,6 @@ class FakeMailboxRepositoryWithInbox implements MailboxRepository {
   Future<int> syncMailboxes(String id) async => 1;
   @override
   Future<Mailbox?> findMailboxByRole(String id, String role) async => null;
+  @override
+  Future<void> clearForResync(String accountId) async {}
 }

test/unit/reliability_runner_test.dart

@@ -0,0 +1,348 @@
+import 'dart:async';
+import 'package:fake_async/fake_async.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sharedinbox/core/models/account.dart';
+import 'package:sharedinbox/core/models/email.dart';
+import 'package:sharedinbox/core/models/mailbox.dart';
+import 'package:sharedinbox/core/repositories/account_repository.dart';
+import 'package:sharedinbox/core/repositories/email_repository.dart';
+import 'package:sharedinbox/core/repositories/mailbox_repository.dart';
+import 'package:sharedinbox/core/repositories/sync_log_repository.dart';
+import 'package:sharedinbox/core/sync/account_sync_manager.dart';
+
+// ── helpers ───────────────────────────────────────────────────────────────────
+
+Account _account({String id = 'a1'}) => Account(
+      id: id,
+      displayName: 'Test',
+      email: 'test@example.com',
+      imapHost: 'localhost',
+    );
+
+class _FakeAccounts implements AccountRepository {
+  final List<Account> accounts;
+  _FakeAccounts([Account? account]) : accounts = [account ?? _account()];
+
+  @override
+  Stream<List<Account>> observeAccounts() => Stream.value(accounts);
+  @override
+  Future<Account?> getAccount(String id) async =>
+      accounts.cast<Account?>().firstWhere(
+            (a) => a?.id == id,
+            orElse: () => null,
+          );
+  @override
+  Future<void> addAccount(Account account, String password) async {}
+  @override
+  Future<void> updateAccount(Account account, {String? password}) async {}
+  @override
+  Future<void> removeAccount(String id) async {}
+  @override
+  Future<String> getPassword(String id) async => 'secret';
+}
+
+class _FakeMailboxes implements MailboxRepository {
+  final List<Mailbox> mailboxes;
+  _FakeMailboxes([this.mailboxes = const []]);
+  @override
+  Stream<List<Mailbox>> observeMailboxes(String? accountId) =>
+      Stream.value(mailboxes);
+  @override
+  Future<int> syncMailboxes(String accountId) async => 0;
+  @override
+  Future<Mailbox?> findMailboxByRole(String accountId, String role) async =>
+      null;
+  @override
+  Future<void> clearForResync(String accountId) async {}
+}
+
+class _CountingEmails implements EmailRepository {
+  int syncCount = 0;
+  int wakeUpCount = 0;
+  final Exception? syncError;
+
+  _CountingEmails({this.syncError});
+
+  @override
+  Future<SyncEmailsResult> syncEmails(String accountId, String mailbox) async {
+    syncCount++;
+    if (syncError != null) throw syncError!;
+    return SyncEmailsResult.zero;
+  }
+
+  @override
+  Future<int> wakeUpEmails(String accountId) async {
+    wakeUpCount++;
+    return 0;
+  }
+
+  @override
+  Future<int> flushPendingChanges(String accountId, String password) async => 0;
+  @override
+  Stream<List<Email>> observeEmails(String a, String m) => Stream.value([]);
+  @override
+  Stream<List<EmailThread>> observeThreads(String a, String m) =>
+      Stream.value([]);
+  @override
+  Stream<List<Email>> observeEmailsInThread(String a, String m, String t) =>
+      Stream.value([]);
+  @override
+  Future<Email?> getEmail(String id) async => null;
+  @override
+  Future<EmailBody> getEmailBody(String id) async =>
+      const EmailBody(emailId: '', attachments: []);
+  @override
+  Future<void> setFlag(String id, {bool? seen, bool? flagged}) async {}
+  @override
+  Future<void> moveEmail(String id, String dest) async {}
+  @override
+  Future<String?> deleteEmail(String id) async => null;
+  @override
+  Future<void> sendEmail(String accountId, EmailDraft draft) async {}
+  @override
+  Future<String> downloadAttachment(String id, EmailAttachment att) async => '';
+  @override
+  Future<List<Email>> searchEmails(String a, String m, String q) async => [];
+  @override
+  Future<List<Email>> searchEmailsGlobal(String? a, String q) async => [];
+  @override
+  Future<List<Email>> getEmailsByAddress(String? a, String addr) async => [];
+  @override
+  Stream<List<FailedMutation>> observeFailedMutations(String a) =>
+      Stream.value([]);
+  @override
+  Future<void> discardMutation(int id) async {}
+  @override
+  Future<void> retryMutation(int id) async {}
+  @override
+  Future<bool> cancelPendingChange(String id, String type) async => false;
+  @override
+  Future<void> snoozeEmail(String id, DateTime until) async {}
+  @override
+  Future<void> restoreEmails(List<Email> emails) async {}
+  @override
+  Stream<String> get onChangesQueued => const Stream.empty();
+  @override
+  Stream<void> watchJmapPush(String accountId, String password) =>
+      const Stream.empty();
+  @override
+  Future<ReliabilityResult> verifySyncReliability(
+    String accountId,
+    String mailboxPath,
+  ) async =>
+      ReliabilityResult.healthy;
+  @override
+  Future<void> clearForResync(String accountId) async {}
+}
+
+class _FakeSyncLog implements SyncLogRepository {
+  final logs = <bool>[];
+  @override
+  Future<void> log({
+    required String accountId,
+    required bool success,
+    String? errorMessage,
+    required String protocol,
+    required int emailsFetched,
+    required int emailsSkipped,
+    required int mailboxesSynced,
+    required int pendingFlushed,
+    required int bytesTransferred,
+    required DateTime startedAt,
+    required DateTime finishedAt,
+    List<MailboxSyncStats> mailboxStats = const [],
+    String? protocolLog,
+  }) async {
+    logs.add(success);
+  }
+
+  @override
+  Stream<List<SyncLogEntry>> observeSyncLogs(String accountId) =>
+      Stream.value([]);
+
+  @override
+  Stream<String?> observeLastError(String accountId) => Stream.value(null);
+}
+
+// ── tests ─────────────────────────────────────────────────────────────────────
+
+void main() {
+  group('AccountSyncManager backoff', () {
+    test('backoff is capped at 900 s after repeated failures', () {
+      fakeAsync((async) {
+        final emails = _CountingEmails(
+          syncError: Exception('connection refused'),
+        );
+        final syncLog = _FakeSyncLog();
+        final manager = AccountSyncManager(
+          _FakeAccounts(),
+          _FakeMailboxes([
+            const Mailbox(
+              id: 'INBOX',
+              accountId: 'a1',
+              path: 'INBOX',
+              name: 'Inbox',
+              unreadCount: 0,
+              totalCount: 0,
+            ),
+          ]),
+          emails,
+          syncLog: syncLog,
+          imapConnect: (_, __, ___) async =>
+              throw Exception('connection refused'),
+        );
+
+        manager.start();
+
+        // Advance 3 hours — long enough to observe many retries.
+        // With max backoff 900 s, we expect at least floor(3*3600/900) = 12
+        // attempts, and at most 3*3600/5 = 2160 (if backoff never grew).
+        async.elapse(const Duration(hours: 3));
+
+        final failCount = syncLog.logs.where((ok) => !ok).length;
+        expect(
+          failCount,
+          greaterThan(10),
+          reason: 'should have retried many times within 3 h',
+        );
+        expect(
+          failCount,
+          lessThan(2200),
+          reason: 'backoff must have kicked in — not every 5 s for 3 h',
+        );
+
+        manager.dispose();
+        async.elapse(const Duration(seconds: 1));
+      });
+    });
+
+    test('backoff resets to 5 s after a successful sync', () {
+      fakeAsync((async) {
+        int callCount = 0;
+        final syncLog = _FakeSyncLog();
+
+        var failsLeft = 5;
+        final customEmails = _OverrideEmails(
+          onSync: (_) async {
+            callCount++;
+            if (failsLeft > 0) {
+              failsLeft--;
+              throw Exception('transient error');
+            }
+            return SyncEmailsResult.zero;
+          },
+        );
+
+        final manager = AccountSyncManager(
+          _FakeAccounts(),
+          _FakeMailboxes([
+            const Mailbox(
+              id: 'INBOX',
+              accountId: 'a1',
+              path: 'INBOX',
+              name: 'Inbox',
+              unreadCount: 0,
+              totalCount: 0,
+            ),
+          ]),
+          customEmails,
+          syncLog: syncLog,
+          imapConnect: (_, __, ___) async =>
+              throw Exception('skip idle — force immediate loop'),
+        );
+
+        manager.start();
+
+        // Allow errors + backoff to build up, then a success, then more loops.
+        async.elapse(const Duration(seconds: 3600));
+
+        // After success, backoff should reset; failures before success should
+        // be exactly 5, and subsequent loops should fire frequently.
+        final successCount = syncLog.logs.where((ok) => ok).length;
+        expect(
+          successCount,
+          greaterThan(0),
+          reason: 'should have at least one success',
+        );
+        expect(
+          callCount,
+          greaterThan(5),
+          reason: 'should retry after failures and continue after success',
+        );
+
+        manager.dispose();
+        async.elapse(const Duration(seconds: 1));
+      });
+    });
+
+    test('concurrent sync errors from multiple accounts stay bounded', () {
+      fakeAsync((async) {
+        final accounts = _FakeAccounts()
+          ..accounts.add(_account(id: 'a2'))
+          ..accounts.add(_account(id: 'a3'));
+        final syncLog = _FakeSyncLog();
+        final manager = AccountSyncManager(
+          accounts,
+          _FakeMailboxes([
+            const Mailbox(
+              id: 'INBOX',
+              accountId: 'a1',
+              path: 'INBOX',
+              name: 'Inbox',
+              unreadCount: 0,
+              totalCount: 0,
+            ),
+            const Mailbox(
+              id: 'INBOX',
+              accountId: 'a2',
+              path: 'INBOX',
+              name: 'Inbox',
+              unreadCount: 0,
+              totalCount: 0,
+            ),
+            const Mailbox(
+              id: 'INBOX',
+              accountId: 'a3',
+              path: 'INBOX',
+              name: 'Inbox',
+              unreadCount: 0,
+              totalCount: 0,
+            ),
+          ]),
+          _CountingEmails(syncError: Exception('network error')),
+          syncLog: syncLog,
+          imapConnect: (_, __, ___) async =>
+              throw Exception('connection refused'),
+        );
+
+        manager.start();
+        async.elapse(const Duration(hours: 2));
+
+        // All 3 accounts retry, each bounded by the 900 s cap.
+        final failCount = syncLog.logs.where((ok) => !ok).length;
+        expect(failCount, greaterThan(5));
+        expect(
+          failCount,
+          lessThan(5000),
+          reason: 'backoff must be in effect across all accounts',
+        );
+
+        manager.dispose();
+        async.elapse(const Duration(seconds: 1));
+      });
+    });
+  });
+}
+
+// ── _OverrideEmails ───────────────────────────────────────────────────────────
+
+class _OverrideEmails extends _CountingEmails {
+  _OverrideEmails({required Future<SyncEmailsResult> Function(String) onSync})
+      : _onSync = onSync;
+
+  final Future<SyncEmailsResult> Function(String) _onSync;
+
+  @override
+  Future<SyncEmailsResult> syncEmails(String accountId, String mailbox) =>
+      _onSync(mailbox);
+}

test/widget/helpers.dart

@@ -132,6 +132,8 @@ class FakeMailboxRepository implements MailboxRepository {
   @override
   Future<Mailbox?> findMailboxByRole(String accountId, String role) async =>
       _mailboxes.where((m) => m.role == role).firstOrNull;
+  @override
+  Future<void> clearForResync(String accountId) async {}
 }
 
 class FakeEmailRepository implements EmailRepository {
@@ -279,6 +281,9 @@ class FakeEmailRepository implements EmailRepository {
 
   @override
   Future<void> retryMutation(int id) async {}
+
+  @override
+  Future<void> clearForResync(String accountId) async {}
 }
 
 // ---------------------------------------------------------------------------