feat: MX record fallback in account auto-discovery

When JMAP well-known and autoconfig XML both fail, query DNS-over-HTTPS
(dns.google) for MX records and use the highest-priority MX host as IMAP
(993/SSL) and SMTP (587/STARTTLS) server. No new dependencies needed.

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

done.md

@@ -6,6 +6,14 @@ Tasks get moved from next.md to done.md
 
 ## Tasks
 
+## MX record fallback in account auto-discovery
+
+When JMAP well-known and autoconfig XML both fail, `AccountDiscoveryServiceImpl` now
+queries `https://dns.google/resolve?name={domain}&type=MX` (DNS-over-HTTPS, no new
+dependency). The highest-priority MX hostname is used as both IMAP host (port 993, SSL)
+and SMTP host (port 587, STARTTLS). Three unit tests cover: basic MX hit, priority
+sorting, and NXDOMAIN/error fallback to `UnknownDiscovery`.
+
 ## Email filters accessible from inside an account
 
 Added "Email filters" entry to `FolderDrawer` (below "All accounts", above the folder

lib/core/services/account_discovery_service.dart

@@ -1,3 +1,5 @@
+import 'dart:convert';
+
 import 'package:http/http.dart' as http;
 
 import 'package:sharedinbox/core/models/discovery_result.dart';
@@ -23,6 +25,9 @@ class AccountDiscoveryServiceImpl implements AccountDiscoveryService {
     final imap = await _tryImapAutoconfig(domain);
     if (imap != null) return imap;
 
+    final mx = await _tryMxFallback(domain);
+    if (mx != null) return mx;
+
     return UnknownDiscovery();
   }
 
@@ -101,4 +106,50 @@ class AccountDiscoveryServiceImpl implements AccountDiscoveryService {
 
   String? _tag(String block, String tag) =>
       RegExp('<$tag>([^<]+)</$tag>').firstMatch(block)?.group(1)?.trim();
+
+  /// Queries DNS-over-HTTPS for MX records and returns an [ImapSmtpDiscovery]
+  /// using the highest-priority MX host with standard ports (IMAP 993/SSL,
+  /// SMTP 587/STARTTLS). Used as a last-resort fallback when neither the JMAP
+  /// well-known endpoint nor the autoconfig XML was found.
+  Future<ImapSmtpDiscovery?> _tryMxFallback(String domain) async {
+    try {
+      final url = Uri.https(
+        'dns.google',
+        '/resolve',
+        {'name': domain, 'type': 'MX'},
+      );
+      final resp = await _client.get(url).timeout(const Duration(seconds: 5));
+      if (resp.statusCode != 200) return null;
+
+      final body = jsonDecode(resp.body) as Map<String, dynamic>;
+      if (body['Status'] != 0) return null;
+
+      final answers = (body['Answer'] as List?)?.cast<Map<String, dynamic>>();
+      if (answers == null || answers.isEmpty) return null;
+
+      final entries = <MapEntry<int, String>>[];
+      for (final a in answers.where((a) => a['type'] == 15)) {
+        final parts = (a['data'] as String).trim().split(RegExp(r'\s+'));
+        if (parts.length < 2) continue;
+        final priority = int.tryParse(parts[0]);
+        if (priority == null) continue;
+        final host = parts[1].replaceAll(RegExp(r'\.$'), '');
+        if (host.isNotEmpty) entries.add(MapEntry(priority, host));
+      }
+      if (entries.isEmpty) return null;
+      entries.sort((a, b) => a.key.compareTo(b.key));
+      final host = entries.first.value;
+
+      return ImapSmtpDiscovery(
+        imapHost: host,
+        imapPort: 993,
+        imapSsl: true,
+        smtpHost: host,
+        smtpPort: 587,
+        smtpSsl: false,
+      );
+    } catch (_) {
+      return null;
+    }
+  }
 }

next.md

@@ -18,11 +18,6 @@ Then commit.
 
 ## Tasks
 
-When adding a new account, and no well-known file was found, not exact hint in DNS, then
-SMTP/IMAP/JMAP should use the mx record as fallback.
-
----
-
 How can I edit Sieve Scripts? Afaik this feature was added.
 
 ---

test/unit/account_discovery_service_test.dart

@@ -119,5 +119,50 @@ void main() {
       final result = await svc.discover('user@example.com');
       expect(result, isA<UnknownDiscovery>());
     });
+
+    test('returns ImapSmtpDiscovery from MX record when autoconfig not found',
+        () async {
+      final svc = _service({
+        'https://dns.google/resolve?name=example.com&type=MX': http.Response(
+          '{"Status":0,"Answer":[{"type":15,"data":"10 mail.example.com."}]}',
+          200,
+        ),
+      });
+      final result = await svc.discover('user@example.com');
+      expect(result, isA<ImapSmtpDiscovery>());
+      final imap = result as ImapSmtpDiscovery;
+      expect(imap.imapHost, 'mail.example.com');
+      expect(imap.imapPort, 993);
+      expect(imap.imapSsl, isTrue);
+      expect(imap.smtpHost, 'mail.example.com');
+      expect(imap.smtpPort, 587);
+      expect(imap.smtpSsl, isFalse);
+    });
+
+    test('MX fallback picks lowest priority record', () async {
+      final svc = _service({
+        'https://dns.google/resolve?name=example.com&type=MX': http.Response(
+          '{"Status":0,"Answer":['
+          '{"type":15,"data":"20 backup.example.com."},'
+          '{"type":15,"data":"10 mail.example.com."}'
+          ']}',
+          200,
+        ),
+      });
+      final result = await svc.discover('user@example.com');
+      expect(result, isA<ImapSmtpDiscovery>());
+      expect((result as ImapSmtpDiscovery).imapHost, 'mail.example.com');
+    });
+
+    test('returns UnknownDiscovery when MX lookup also fails', () async {
+      final svc = _service({
+        'https://dns.google/resolve?name=example.com&type=MX': http.Response(
+          '{"Status":3}',
+          200,
+        ),
+      });
+      final result = await svc.discover('user@example.com');
+      expect(result, isA<UnknownDiscovery>());
+    });
   });
 }