TF-1677: Add test case

(cherry picked from commit c0fa82c8f919786b17605a0ea066ea8353a8e0aa)

test/main/utils/app_utils_test.dart

@@ -0,0 +1,37 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:jmap_dart_client/jmap/mail/email/email_address.dart';
+import 'package:model/model.dart';
+import 'package:tmail_ui_user/main/utils/app_utils.dart';
+
+void main() {
+  group('Validation email address test', () {
+
+    final emailsValid = [
+      EmailAddress("userName", "alice@localhost"),
+      EmailAddress("userName", "bob@localhost"),
+      EmailAddress("userName", "123@localhost"),
+      EmailAddress("userName", "alic.2312e@localhost"),
+      EmailAddress("userName", "alice_linagora@localhost")
+    ];
+
+    final emailsInvalid = [
+      EmailAddress("userName", "localhost"),
+      EmailAddress("userName", "bob@local"),
+      EmailAddress("userName", "123@domain"),
+      EmailAddress("userName", "alic.2312e@"),
+      EmailAddress("userName", "a,.2312lic.2312e@")
+    ];
+
+    test('Valid localhost email addresses', () {
+      for (final email in emailsValid) {
+        expect(AppUtils.isEmailLocalhost(email.emailAddress), equals(true));
+      }
+    });
+
+    test('Invalid localhost email addresses', () {
+      for (final email in emailsInvalid) {
+        expect(AppUtils.isEmailLocalhost(email.emailAddress), equals(false));
+      }
+    });
+  });
+}