diff --git a/addons/mail/models/mail_message.py b/addons/mail/models/mail_message.py
index 1ced8572620..b6df0852c87 100644
--- a/addons/mail/models/mail_message.py
+++ b/addons/mail/models/mail_message.py
@@ -884,7 +884,8 @@ class Message(models.Model):
             if message_sudo.model and message_sudo.res_id:
                 record_sudo = self.env[message_sudo.model].browse(message_sudo.res_id).sudo()
                 record_name = record_sudo.with_prefetch(thread_ids_by_model_name[message_sudo.model]).display_name
-                default_subject = record_sudo._message_compute_subject()
+                # if not a thread, display_name is the default subject
+                default_subject = record_sudo._message_compute_subject() if hasattr(record_sudo, '_message_compute_subject') else record_name
             else:
                 record_name = False
                 default_subject = False
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 279631b4403..6f0fab85923 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -3347,7 +3347,7 @@ class MailThread(models.AbstractModel):
         :return: dictionary of values suitable for a <mail.mail> create;
         """
         mail_subject = message.subject
-        if not mail_subject and self:
+        if not mail_subject and self and hasattr(self, '_message_compute_subject'):
             mail_subject = self._message_compute_subject()
         if not mail_subject:
             mail_subject = message.record_name
diff --git a/addons/mail/wizard/mail_compose_message.py b/addons/mail/wizard/mail_compose_message.py
index cbd83e193ae..c9213eb02e5 100644
--- a/addons/mail/wizard/mail_compose_message.py
+++ b/addons/mail/wizard/mail_compose_message.py
@@ -195,8 +195,8 @@ class MailComposer(models.TransientModel):
         for composer in self:
             composer._evaluate_res_domain()
 
-    @api.depends('composition_mode', 'model', 'parent_id', 'res_domain',
-                 'res_ids', 'template_id')
+    @api.depends('composition_mode', 'model', 'parent_id', 'record_name',
+                 'res_domain', 'res_ids', 'template_id')
     def _compute_subject(self):
         """ Computation is coming either form template, either from context.
         When having a template with a value set, copy it (in batch mode) or
@@ -212,8 +212,11 @@ class MailComposer(models.TransientModel):
                 if (not subject and composer.model and
                     composer.composition_mode == 'comment' and
                     not composer.composition_batch):
-                    res_ids = composer._evaluate_res_ids()
-                    subject = self.env[composer.model].browse(res_ids)._message_compute_subject()
+                    if composer.model_is_thread:
+                        res_ids = composer._evaluate_res_ids()
+                        subject = self.env[composer.model].browse(res_ids)._message_compute_subject()
+                    else:
+                        subject = composer.record_name
                 composer.subject = subject
 
     @api.depends('composition_mode', 'model', 'res_domain', 'res_ids',
diff --git a/addons/test_mail/tests/test_mail_composer.py b/addons/test_mail/tests/test_mail_composer.py
index aa46c1aa3f6..b74993a726b 100644
--- a/addons/test_mail/tests/test_mail_composer.py
+++ b/addons/test_mail/tests/test_mail_composer.py
@@ -1201,34 +1201,41 @@ class TestComposerResultsComment(TestMailComposer, CronMixinCase):
     @users('employee')
     def test_mail_composer_default_subject(self):
         """ Make sure the default subject is applied in the composer. """
-        simple_record = self.env['mail.test.simple'].create({'name': 'TestSimple'})
-        ticket_record = self.env['mail.test.ticket'].create({'name': 'TestTicket'})
+        simple_record = self.env['mail.test.simple'].create({'name': 'TestA'})
+        ticket_record = self.env['mail.test.ticket'].create({'name': 'Test1'})
+        nonthread_record = self.env['mail.test.nothread'].create({'name': 'TestNoThread'})
 
-        # default behavior: use record name
-        _, message = self.env['mail.compose.message'].with_context(
-            self._get_web_context(simple_record, add_web=False, composition_mode='comment')
-        ).create({
+        # default behavior: use the record name
+        ctx = self._get_web_context(simple_record, add_web=False, default_composition_mode='comment')
+        _, message = self.env['mail.compose.message'].with_context(ctx).create({
             'body': '<p>Test Body</p>',
         })._action_send_mail()
         self.assertEqual(message.subject, simple_record.name)
 
-        # overridden in model
-        _, message = self.env['mail.compose.message'].with_context(
-            self._get_web_context(ticket_record, add_web=False, composition_mode='comment')
-        ).create({
+        # default behavior without thread: use record name
+        ctx = self._get_web_context(nonthread_record, add_web=False, default_composition_mode='comment')
+        _, message = self.env['mail.compose.message'].with_context(ctx).create({
+            'body': '<p>Test Body</p>',
+            'partner_ids': self.env.user.partner_id.ids,
+        })._action_send_mail()
+        self.assertEqual(message.record_name, nonthread_record.name)
+        self.assertEqual(message.subject, nonthread_record.name)
+
+        # custom subject
+        ctx = self._get_web_context(ticket_record, add_web=False, default_composition_mode='comment')
+        _, message = self.env['mail.compose.message'].with_context(ctx).create({
             'body': '<p>Test Body</p>',
         })._action_send_mail()
         self.assertEqual(message.subject, ticket_record._message_compute_subject())
-        self.assertIn(f'Ticket for {ticket_record.name}', message.subject)
+        self.assertEqual(message.subject, f'Ticket for {ticket_record.name} on {ticket_record.datetime.strftime("%m/%d/%Y, %H:%M:%S")}')
 
         # forced value
-        _, message = self.env['mail.compose.message'].with_context(
-            self._get_web_context(ticket_record, add_web=False, composition_mode='comment')
-        ).create({
+        ctx = self._get_web_context(ticket_record, add_web=False, default_composition_mode='comment')
+        _, message = self.env['mail.compose.message'].with_context(ctx).create({
             'body': '<p>Test Body</p>',
             'subject': 'Forced Subject',
         })._action_send_mail()
-        self.assertEqual(message.subject, 'Forced Subject',)
+        self.assertEqual(message.subject, 'Forced Subject')
 
     @users('employee')
     @mute_logger('odoo.tests', 'odoo.addons.mail.models.mail_mail', 'odoo.models.unlink')
diff --git a/addons/test_mail/tests/test_mail_message.py b/addons/test_mail/tests/test_mail_message.py
index 8530d41111d..d04e3aec4f6 100644
--- a/addons/test_mail/tests/test_mail_message.py
+++ b/addons/test_mail/tests/test_mail_message.py
@@ -97,16 +97,18 @@ class TestMessageValues(MailCommon):
     @mute_logger('odoo.models.unlink')
     def test_mail_message_format(self):
         record1 = self.env['mail.test.simple'].create({'name': 'Test1'})
-        message = self.env['mail.message'].create([{
-            'model': 'mail.test.simple',
-            'res_id': record1.id,
-        }])
-        res = message.message_format()
-        self.assertEqual(res[0].get('record_name'), 'Test1')
-
-        record1.write({"name": "Test2"})
-        res = message.message_format()
-        self.assertEqual(res[0].get('record_name'), 'Test2')
+        record2 = self.env['mail.test.nothread'].create({'name': 'Test2'})
+        messages = self.env['mail.message'].create([{
+            'model': record._name,
+            'res_id': record.id,
+        } for record in [record1, record2]])
+        for message, record in zip(messages, [record1, record2]):
+            with self.subTest(record=record):
+                formatted = message.message_format()[0]
+                self.assertEqual(formatted['record_name'], record.name)
+                record.write({'name': 'Just Test'})
+                formatted = message.message_format()[0]
+                self.assertEqual(formatted['record_name'], 'Just Test')
 
     @mute_logger('odoo.models.unlink')
     def test_mail_message_format_access(self):
diff --git a/addons/test_mail/tests/test_mail_thread_internals.py b/addons/test_mail/tests/test_mail_thread_internals.py
index de325bdfcb3..4bafa0458c5 100644
--- a/addons/test_mail/tests/test_mail_thread_internals.py
+++ b/addons/test_mail/tests/test_mail_thread_internals.py
@@ -404,36 +404,63 @@ class TestNoThread(MailCommon, TestRecipients):
     """ Specific tests for cross models thread features """
 
     @users('employee')
-    def test_message_notify(self):
-        """ Test notifying using model / res_id linking to a model not being
-        mail.thread enabled. """
+    def test_message_format(self):
+        """ Test formatting of messages when linked to non-thread models.
+        Format could be asked notably if an inbox notification due to a
+        'message_notify' happens. """
         test_record = self.env['mail.test.nothread'].create({
             'customer_id': self.partner_1.id,
             'name': 'Not A Thread',
         })
-        with self.assertPostNotifications([{
-                'content': 'Hello Paulo',
-                'email_values': {
-                    'reply_to': self.company_admin.catchall_formatted,
-                    'subject': 'Test Notify',
-                },
-                'message_type': 'user_notification',
-                'notif': [{
-                    'check_send': True,
-                    'is_read': True,
-                    'partner': self.partner_2,
-                    'status': 'sent',
-                    'type': 'email',
-                }],
-                'subtype': 'mail.mt_note',
-            }]):
-            _message = self.env['mail.thread'].message_notify(
-                body='<p>Hello Paulo</p>',
-                model=test_record._name,
-                partner_ids=self.partner_2.ids,
-                res_id=test_record.id,
-                subject='Test Notify',
-            )
+        message = self.env['mail.message'].create({
+            'model': test_record._name,
+            'record_name': 'Not used in message_format',
+            'res_id': test_record.id,
+        })
+        formatted = message.message_format()[0]
+        self.assertEqual(formatted['default_subject'], test_record.name)
+        self.assertEqual(formatted['record_name'], test_record.name)
+
+        test_record.write({'name': 'Just Test'})
+        formatted = message.message_format()[0]
+        self.assertEqual(formatted['default_subject'], 'Just Test')
+        self.assertEqual(formatted['record_name'], 'Just Test')
+
+    @users('employee')
+    def test_message_notify(self):
+        """ Test notifying on non-thread models, using MailThread as an abstract
+        class with model and res_id giving the record used for notification.
+
+        Test default subject computation is also tested. """
+        test_record = self.env['mail.test.nothread'].create({
+            'customer_id': self.partner_1.id,
+            'name': 'Not A Thread',
+        })
+
+        for subject in ["Test Notify", False]:
+            with self.subTest():
+                with self.assertPostNotifications([{
+                        'content': 'Hello Paulo',
+                        'email_values': {
+                            'reply_to': self.company_admin.catchall_formatted,
+                        },
+                        'message_type': 'user_notification',
+                        'notif': [{
+                            'check_send': True,
+                            'is_read': True,
+                            'partner': self.partner_2,
+                            'status': 'sent',
+                            'type': 'email',
+                        }],
+                        'subtype': 'mail.mt_note',
+                    }]):
+                    _message = self.env['mail.thread'].message_notify(
+                        body='<p>Hello Paulo</p>',
+                        model=test_record._name,
+                        partner_ids=self.partner_2.ids,
+                        res_id=test_record.id,
+                        subject=subject,
+                    )
 
     @users('employee')
     def test_message_notify_composer(self):