diff --git a/addons/mail/tests/common.py b/addons/mail/tests/common.py
index 1311678506e..1d79a301d87 100644
--- a/addons/mail/tests/common.py
+++ b/addons/mail/tests/common.py
@@ -6,6 +6,7 @@ import email
 import email.policy
 import time
 
+from ast import literal_eval
 from collections import defaultdict
 from contextlib import contextmanager
 from functools import partial
@@ -398,12 +399,16 @@ class MockEmail(common.BaseCase, MockSmtplibCase):
         self.assertTrue(bool(mail))
         if content:
             self.assertIn(content, mail.body_html)
-        for fname, fvalue in (fields_values or {}).items():
-            with self.subTest(fname=fname, fvalue=fvalue):
-                self.assertEqual(
-                    mail[fname], fvalue,
-                    'Mail: expected %s for %s, got %s' % (fvalue, fname, mail[fname])
-                )
+        for fname, expected_fvalue in (fields_values or {}).items():
+            with self.subTest(fname=fname, expected_fvalue=expected_fvalue):
+                if fname == 'headers':
+                    fvalue = literal_eval(mail[fname])
+                    self.assertDictEqual(fvalue, expected_fvalue)
+                else:
+                    self.assertEqual(
+                        mail[fname], expected_fvalue,
+                        'Mail: expected %s for %s, got %s' % (expected_fvalue, fname, mail[fname])
+                    )
         if status == 'sent':
             if email_to_recipients:
                 recipients = email_to_recipients  # already formatted
diff --git a/addons/test_mail/tests/test_mail_alias.py b/addons/test_mail/tests/test_mail_alias.py
index 5e6c7603998..5f4e2002827 100644
--- a/addons/test_mail/tests/test_mail_alias.py
+++ b/addons/test_mail/tests/test_mail_alias.py
@@ -293,6 +293,7 @@ class TestMailAliasMixin(TestMailAliasCommon):
     @users('employee')
     @mute_logger('odoo.addons.base.models.ir_model')
     def test_alias_mixin(self):
+        """ Various base checks on alias mixin behavior """
         record = self.env['mail.test.container'].create({
             'name': 'Test Record',
             'alias_name': 'alias.test',
@@ -380,8 +381,10 @@ class TestMailAliasMixin(TestMailAliasCommon):
         self.assertFalse(list(filter(None, existing_aliases.mapped('alias_name'))))
 
     @users('employee')
-    def test_alias_mixin_copy_content(self):
-        self.assertFalse(self.env.user.has_group('base.group_system'), 'Test user should not have Administrator access')
+    def test_copy_content(self):
+        self.assertFalse(
+            self.env.user.has_group('base.group_system'),
+            'Test user should not have Administrator access')
 
         record = self.env['mail.test.container'].create({
             'name': 'Test Record',
@@ -389,9 +392,16 @@ class TestMailAliasMixin(TestMailAliasCommon):
             'alias_contact': 'followers',
             'alias_bounced_content': False,
         })
+        record_alias = record.alias_id
         self.assertFalse(record.alias_bounced_content)
         record_copy = record.copy()
+        record_alias_copy = record_copy.alias_id
+        self.assertNotEqual(record_alias, record_alias_copy)
+        self.assertEqual(record_alias.alias_force_thread_id, record.id)
+        self.assertEqual(record_alias_copy.alias_force_thread_id, record_copy.id)
         self.assertFalse(record_copy.alias_bounced_content)
+        self.assertEqual(record_copy.alias_contact, record.alias_contact)
+        self.assertFalse(record_copy.alias_name, 'Copy should not duplicate name')
 
         new_content = '<p>Bounced Content</p>'
         record_copy.write({'alias_bounced_content': new_content})
@@ -400,7 +410,7 @@ class TestMailAliasMixin(TestMailAliasCommon):
         self.assertEqual(record_copy2.alias_bounced_content, new_content)
 
     @users('erp_manager')
-    def test_alias_mixin_mc(self):
+    def test_multi_company_setup(self):
         """ Test company change does not impact anything at alias domain level """
         record = self.env['mail.test.container.mc'].create({
             'name': 'Test Record',
diff --git a/addons/test_mail/tests/test_mail_gateway.py b/addons/test_mail/tests/test_mail_gateway.py
index 344ebbfbf7e..2bbe767d4f3 100644
--- a/addons/test_mail/tests/test_mail_gateway.py
+++ b/addons/test_mail/tests/test_mail_gateway.py
@@ -929,35 +929,27 @@ class TestMailgateway(MailCommon):
         """ Incoming email: check that if domains are set in the optional system
         parameter `mail.catchall.domain.allowed` only incoming emails from these
         domains will generate records."""
-        MailTestGatewayModel = self.env['mail.test.gateway']
-        MailTestContainerModel = self.env['mail.test.container']
-
         # test@.. will cause the creation of new mail.test
         new_alias_2 = self.env['mail.alias'].create({
             'alias_contact': 'everyone',
-            'alias_model_id': self.env['ir.model']._get('mail.test.container').id,
+            'alias_model_id': self.env['ir.model']._get_id('mail.test.container.mc'),
             'alias_name': 'test',
             'alias_user_id': False,
         })
 
-        allowed_domain = 'hello.com'
-        for (alias_right_part, allowed_domain), (gateway_created, container_created) in zip(
+        test_domain = 'hello.com'
+        for (alias_right_part, allowed_domain), container_created in zip(
             [
                 # Test with 'mail.catchall.domain.allowed' not set in system parameters
                 # and with a domain not allowed
                 ('bonjour.com', ""),
                 # Test with 'mail.catchall.domain.allowed' set in system parameters
                 # and with a domain not allowed
-                ('bonjour.com', allowed_domain),
+                ('bonjour.com', test_domain),
                 # Test with 'mail.catchall.domain.allowed' set in system parameters
                 # and with a domain allowed
-                (allowed_domain, allowed_domain),
-            ], [
-                (True, True),
-                (True, False),
-                (True, True),
-            ]
-        ):
+                (test_domain, test_domain),
+            ], [True, False, True]):
             with self.subTest(alias_right_part=alias_right_part, allowed_domain=allowed_domain):
                 self.env['ir.config_parameter'].set_param('mail.catchall.domain.allowed', allowed_domain)
 
@@ -970,9 +962,9 @@ class TestMailgateway(MailCommon):
                     target_model=self.alias.alias_model_id.model
                 )
 
-                res_alias_1 = MailTestGatewayModel.search([('name', '=', subject)])
-                res_alias_2 = MailTestContainerModel.search([('name', '=', subject)])
-                self.assertEqual(bool(res_alias_1), gateway_created)
+                res_alias_1 = self.env['mail.test.gateway'].search([('name', '=', subject)])
+                res_alias_2 = self.env['mail.test.container.mc'].search([('name', '=', subject)])
+                self.assertTrue(bool(res_alias_1), 'First alias should always be respected')
                 self.assertEqual(bool(res_alias_2), container_created)
 
     # --------------------------------------------------
diff --git a/addons/test_mail/tests/test_message_post.py b/addons/test_mail/tests/test_message_post.py
index d4c396cd876..8147c4bf832 100644
--- a/addons/test_mail/tests/test_message_post.py
+++ b/addons/test_mail/tests/test_message_post.py
@@ -969,9 +969,9 @@ class TestMessagePost(TestMessagePostCommon, CronMixinCase):
                                 },
                             },
                             'mail_mail_values': {
-                                'headers': repr({
+                                'headers': {
                                     'X-Odoo-Objects': f'{record._name}-{record.id}',
-                                }),
+                                },
                             },
                             'message_values': {
                                 'author_id': self.user_erp_manager.partner_id,