feat(U1): show Unsubscribe chip for emails with List-Unsubscribe header

- Add listUnsubscribeHeader nullable text column to Emails table (schema v23)
- Parse List-Unsubscribe header from IMAP (BODY.PEEK[HEADER.FIELDS]) and JMAP (header:List-Unsubscribe:asText property)
- Show ActionChip in EmailDetailScreen that launches the unsubscribe URI (prefers mailto:, falls back to https:)

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

lib/core/models/email.dart

@@ -21,6 +21,8 @@ class Email {
   final String? references;
   final DateTime? snoozedUntil;
   final String? snoozedFromMailboxPath;
+  // RFC 2369 List-Unsubscribe header value, e.g. "<mailto:...>, <https://...>".
+  final String? listUnsubscribeHeader;
 
   const Email({
     required this.id,
@@ -43,6 +45,7 @@ class Email {
     this.references,
     this.snoozedUntil,
     this.snoozedFromMailboxPath,
+    this.listUnsubscribeHeader,
   });
 
   factory Email.fromJson(Map<String, dynamic> json) {
@@ -77,6 +80,7 @@ class Email {
           ? DateTime.parse(json['snoozedUntil'] as String)
           : null,
       snoozedFromMailboxPath: json['snoozedFromMailboxPath'] as String?,
+      listUnsubscribeHeader: json['listUnsubscribeHeader'] as String?,
     );
   }
 
@@ -102,6 +106,7 @@ class Email {
       'references': references,
       'snoozedUntil': snoozedUntil?.toIso8601String(),
       'snoozedFromMailboxPath': snoozedFromMailboxPath,
+      'listUnsubscribeHeader': listUnsubscribeHeader,
     };
   }
 
@@ -126,6 +131,7 @@ class Email {
     String? references,
     DateTime? snoozedUntil,
     String? snoozedFromMailboxPath,
+    String? listUnsubscribeHeader,
   }) {
     return Email(
       id: id ?? this.id,
@@ -149,6 +155,8 @@ class Email {
       snoozedUntil: snoozedUntil ?? this.snoozedUntil,
       snoozedFromMailboxPath:
           snoozedFromMailboxPath ?? this.snoozedFromMailboxPath,
+      listUnsubscribeHeader:
+          listUnsubscribeHeader ?? this.listUnsubscribeHeader,
     );
   }
 }

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/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/db/database.dart

@@ -88,6 +88,9 @@ class Emails extends Table {
   DateTimeColumn get snoozedUntil => dateTime().nullable()();
   TextColumn get snoozedFromMailboxPath => text().nullable()();
 
+  // Added in schema v23: RFC 2369 List-Unsubscribe header value.
+  TextColumn get listUnsubscribeHeader => text().nullable()();
+
   @override
   Set<Column> get primaryKey => {id};
 }
@@ -264,7 +267,7 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase([QueryExecutor? executor]) : super(executor ?? _openConnection());
 
   @override
-  int get schemaVersion => 22;
+  int get schemaVersion => 23;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -420,6 +423,9 @@ class AppDatabase extends _$AppDatabase {
               ),
             );
           }
+          if (from < 23) {
+            await m.addColumn(emails, emails.listUnsubscribeHeader);
+          }
         },
       );
 }

lib/data/repositories/email_repository_impl.dart

@@ -528,7 +528,7 @@ class EmailRepositoryImpl implements EmailRepository {
     imap.MessageSequence sequence,
   ) async {
     const fetchItems =
-        '(UID FLAGS ENVELOPE BODYSTRUCTURE RFC822.SIZE BODY.PEEK[HEADER.FIELDS (REFERENCES)])';
+        '(UID FLAGS ENVELOPE BODYSTRUCTURE RFC822.SIZE BODY.PEEK[HEADER.FIELDS (REFERENCES LIST-UNSUBSCRIBE)])';
     final fetch = sequence.isUidSequence
         ? await client.uidFetchMessages(sequence, fetchItems)
         : await client.fetchMessages(sequence, fetchItems);
@@ -569,6 +569,7 @@ class EmailRepositoryImpl implements EmailRepository {
         final msgId = envelope.messageId?.trim();
         final inReplyTo = envelope.inReplyTo?.trim();
         final refs = msg.getHeaderValue('References')?.trim();
+        final listUnsubscribe = msg.getHeaderValue('List-Unsubscribe')?.trim();
         final threadId = _computeThreadId(
               emailId: emailId,
               messageId: msgId,
@@ -612,6 +613,7 @@ class EmailRepositoryImpl implements EmailRepository {
                 inReplyTo: Value(inReplyTo),
                 references: Value(refs),
                 snoozedUntil: Value(snoozedUntil),
+                listUnsubscribeHeader: Value(listUnsubscribe),
               ),
             );
       }
@@ -950,6 +952,7 @@ class EmailRepositoryImpl implements EmailRepository {
     'htmlBody',
     'bodyValues',
     'attachments',
+    'header:List-Unsubscribe:asText',
   ];
 
   static const _emailGetBodyOptions = {
@@ -1151,6 +1154,8 @@ class EmailRepositoryImpl implements EmailRepository {
       final jmapReferences = _joinJmapStringList(
         m['references'] as List<dynamic>?,
       );
+      final jmapListUnsubscribe =
+          (m['header:List-Unsubscribe:asText'] as String?)?.trim();
 
       await _db.into(_db.emails).insertOnConflictUpdate(
             EmailsCompanion.insert(
@@ -1173,6 +1178,7 @@ class EmailRepositoryImpl implements EmailRepository {
               inReplyTo: Value(jmapInReplyTo),
               references: Value(jmapReferences),
               snoozedUntil: Value(snoozedUntil),
+              listUnsubscribeHeader: Value(jmapListUnsubscribe),
             ),
           );
 
@@ -2663,6 +2669,7 @@ class EmailRepositoryImpl implements EmailRepository {
       references: row.references,
       snoozedUntil: row.snoozedUntil,
       snoozedFromMailboxPath: row.snoozedFromMailboxPath,
+      listUnsubscribeHeader: row.listUnsubscribeHeader,
     );
   }
 

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

@@ -324,11 +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(
@@ -348,6 +348,7 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
                     _sieveHostCtrl,
                     'Host (leave blank to use IMAP host)',
                     required: false,
+                    validator: validateOptionalHostname,
                   ),
                   _field(
                     _sievePortCtrl,
@@ -408,6 +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),
@@ -420,9 +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_detail_screen.dart

@@ -13,6 +13,7 @@ import 'package:sharedinbox/core/utils/format_utils.dart';
 import 'package:sharedinbox/core/utils/html_utils.dart';
 import 'package:sharedinbox/di.dart';
 import 'package:sharedinbox/ui/widgets/snooze_picker.dart';
+import 'package:url_launcher/url_launcher.dart';
 
 final _dateFmt = DateFormat('EEE, MMM d yyyy, HH:mm');
 
@@ -267,6 +268,11 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
             _dateFmt.format(email.sentAt!),
             style: Theme.of(ctx).textTheme.bodySmall,
           ),
+        if (email.listUnsubscribeHeader != null)
+          Padding(
+            padding: const EdgeInsets.only(top: 8),
+            child: _UnsubscribeChip(header: email.listUnsubscribeHeader!),
+          ),
       ],
     );
   }
@@ -462,6 +468,39 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
   }
 }
 
+/// Parses a List-Unsubscribe header and returns the first usable URI.
+/// Prefers mailto: so unsubscribing sends an email; falls back to https:.
+Uri? _parseUnsubscribeUri(String header) {
+  final matches = RegExp(r'<([^>]+)>').allMatches(header);
+  Uri? fallback;
+  for (final m in matches) {
+    final raw = m.group(1)!.trim();
+    final uri = Uri.tryParse(raw);
+    if (uri == null) continue;
+    if (uri.scheme == 'mailto') return uri;
+    if ((uri.scheme == 'https' || uri.scheme == 'http') && fallback == null) {
+      fallback = uri;
+    }
+  }
+  return fallback;
+}
+
+class _UnsubscribeChip extends StatelessWidget {
+  const _UnsubscribeChip({required this.header});
+  final String header;
+
+  @override
+  Widget build(BuildContext context) {
+    final uri = _parseUnsubscribeUri(header);
+    if (uri == null) return const SizedBox.shrink();
+    return ActionChip(
+      avatar: const Icon(Icons.unsubscribe_outlined, size: 16),
+      label: const Text('Unsubscribe'),
+      onPressed: () => launchUrl(uri, mode: LaunchMode.externalApplication),
+    );
+  }
+}
+
 class _BlockRemoteImagesExtension extends HtmlExtension {
   @override
   Set<String> get supportedTags => {'img'};

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

@@ -220,4 +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

@@ -132,6 +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 {

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);
+}