diff --git a/addons/crm/tests/test_crm_lead_notification.py b/addons/crm/tests/test_crm_lead_notification.py
index 35c7648912cf7..b257fc34d884e 100644
--- a/addons/crm/tests/test_crm_lead_notification.py
+++ b/addons/crm/tests/test_crm_lead_notification.py
@@ -389,11 +389,8 @@ def test_new_lead_from_email_multicompany(self):
 
 --000000000000a47519057e029630--
 """
-        crm_lead0_id = self.env['mail.thread'].message_process('crm.lead', new_message0)
-        crm_lead1_id = self.env['mail.thread'].message_process('crm.lead', new_message1)
-
-        crm_lead0 = self.env['crm.lead'].browse(crm_lead0_id)
-        crm_lead1 = self.env['crm.lead'].browse(crm_lead1_id)
+        crm_lead0 = self.env['mail.thread'].message_process('crm.lead', new_message0)
+        crm_lead1 = self.env['mail.thread'].message_process('crm.lead', new_message1)
 
         self.assertEqual(crm_lead0.team_id, crm_team0)
         self.assertEqual(crm_lead1.team_id, crm_team1)
diff --git a/addons/mail/models/fetchmail.py b/addons/mail/models/fetchmail.py
index b30725e918df1..347280119ce2a 100644
--- a/addons/mail/models/fetchmail.py
+++ b/addons/mail/models/fetchmail.py
@@ -231,11 +231,10 @@ def fetch_mail(self, raise_exception=True):
                     imap_server.select()
                     result, data = imap_server.search(None, '(UNSEEN)')
                     for num in data[0].split():
-                        res_id = None
                         result, data = imap_server.fetch(num, '(RFC822)')
                         imap_server.store(num, '-FLAGS', '\\Seen')
                         try:
-                            res_id = MailThread.with_context(**additionnal_context).message_process(server.object_id.model, data[0][1], save_original=server.original, strip_attachments=(not server.attach))
+                            MailThread.with_context(**additionnal_context).message_process(server.object_id.model, data[0][1], save_original=server.original, strip_attachments=(not server.attach))
                         except Exception:
                             _logger.info('Failed to process mail from %s server %s.', server.server_type, server.name, exc_info=True)
                             failed += 1
@@ -266,9 +265,8 @@ def fetch_mail(self, raise_exception=True):
                         for num in range(1, min(MAX_POP_MESSAGES, num_messages) + 1):
                             (header, messages, octets) = pop_server.retr(num)
                             message = (b'\n').join(messages)
-                            res_id = None
                             try:
-                                res_id = MailThread.with_context(**additionnal_context).message_process(server.object_id.model, message, save_original=server.original, strip_attachments=(not server.attach))
+                                MailThread.with_context(**additionnal_context).message_process(server.object_id.model, message, save_original=server.original, strip_attachments=(not server.attach))
                                 pop_server.dele(num)
                             except Exception:
                                 _logger.info('Failed to process mail from %s server %s.', server.server_type, server.name, exc_info=True)
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 58fb0a655ee9d..2848287780a83 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -1291,7 +1291,7 @@ def _message_route_process(self, message, message_dict, routes):
         self = self.with_context(attachments_mime_plainxml=True) # import XML attachments as text
         # postpone setting message_dict.partner_ids after message_post, to avoid double notifications
         original_partner_ids = message_dict.pop('partner_ids', [])
-        thread_id = False
+        thread = self.browse()
         for model, thread_id, custom_values, user_id, alias in routes or ():
             subtype_id = False
             related_user = self.env['res.users'].browse(user_id)
@@ -1361,9 +1361,10 @@ def _message_route_process(self, message, message_dict, routes):
                 # postponed after message_post, because this is an external message and we don't want to create
                 # duplicate emails due to notifications
                 new_msg.write({'partner_ids': original_partner_ids})
-        return thread_id
+        return thread.with_env(self.env)
 
     @api.model
+    @api.returns('mail.thread', lambda value: value.id)
     def message_process(self, model, message, custom_values=None,
                         save_original=False, strip_attachments=False,
                         thread_id=None):
@@ -1424,8 +1425,7 @@ def message_process(self, model, message, custom_values=None,
         if self._detect_loop_sender(message, msg_dict, routes):
             return
 
-        thread_id = self._message_route_process(message, msg_dict, routes)
-        return thread_id
+        return self._message_route_process(message, msg_dict, routes)
 
     @api.model
     def message_new(self, msg_dict, custom_values=None):