diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 599cec0493c..a733f2f329b 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -1109,37 +1109,23 @@ class MailThread(models.AbstractModel):
                 reply_match = False
 
         if reply_match:
-            compat_mode = False
             msg_references = tools.mail_header_msgid_re.findall(thread_references)
             mail_messages = MailMessage.sudo().search([('message_id', 'in', msg_references)], limit=1)
 
-            # message is a reply to an existing thread (6.1 compatibility)
-            # do not match forwarded emails from another OpenERP system (thread_id collision!)
-            if not mail_messages and local_hostname == reply_hostname and reply_thread_id and reply_model in self.env:
-                mail_messages = MailMessage.search([
-                    ('message_id', '=', False),
-                    ('model', '=', reply_model),
-                    ('res_id', '=', reply_thread_id)])
-                compat_mode = True
-
             if mail_messages:
                 model, thread_id = mail_messages.model, mail_messages.res_id
                 if not reply_private:  # TDE note: not sure why private mode as no alias search, copying existing behavior
                     dest_aliases = Alias.search([('alias_name', 'in', rcpt_tos_localparts)], limit=1)
 
-                # TDE Note: compat mode = without context key, why ? because
                 route = self.message_route_verify(
                     message, message_dict,
                     (model, thread_id, custom_values, self._uid, dest_aliases),
                     update_author=True, assert_model=reply_private, create_fallback=True,
                     allow_private=reply_private, drop_alias=True)
                 if route:
-                    # TDE Note: compat mode: parent is invalid for a compat-reply
-                    # message_dict.pop('parent_id', None)
-                    # TDE note: add compat mode for compat mode in debug
                     _logger.info(
-                        'Routing mail from %s to %s with Message-Id %s%s: direct reply to msg: model: %s, thread_id: %s, custom_values: %s, uid: %s',
-                        email_from, email_to, message_id, compat_mode and ' (compat mode)' or '', model, thread_id, custom_values, self._uid)
+                        'Routing mail from %s to %s with Message-Id %s: direct reply to msg: model: %s, thread_id: %s, custom_values: %s, uid: %s',
+                        email_from, email_to, message_id, model, thread_id, custom_values, self._uid)
                     return [route]
                 elif route is False:
                     return []
diff --git a/addons/mail/tests/test_mail_gateway.py b/addons/mail/tests/test_mail_gateway.py
index 707e316bca5..b729a105b98 100644
--- a/addons/mail/tests/test_mail_gateway.py
+++ b/addons/mail/tests/test_mail_gateway.py
@@ -580,25 +580,29 @@ class TestMailgateway(TestMail):
                           msg_id='<1198923581.41972151344608186802.JavaMail.diff1@agrolait.com>')
 
         # when 6.1 messages are present, compat mode is available
+        # Odoo 10 update: compat mode has been removed and should not work anymore
         self.fake_email.write({'message_id': False})
         # Do: compat mode accepts partial-matching emails
-        frog_groups = self.format_and_process(
+        self.assertRaises(
+            ValueError,
+            self.format_and_process,
             MAIL_TEMPLATE, email_from='other5@gmail.com',
             msg_id='<1.2.JavaMail.new@agrolait.com>',
             to='noone@example.com>', subject='spam',
             extra='In-Reply-To: <12321321-openerp-%d-mail.channel@%s>' % (self.group_public.id, socket.gethostname()))
 
         # 3''. 6.1 compat mode should not work if hostname does not match!
+        # Odoo 10 update: compat mode has been removed and should not work anymore and does not depend from hostname
         self.assertRaises(ValueError,
                           self.format_and_process,
                           MAIL_TEMPLATE, email_from='other5@gmail.com',
                           msg_id='<1.3.JavaMail.new@agrolait.com>',
                           to='noone@example.com>', subject='spam',
-                          extra='In-Reply-To: <12321321-openerp-%d-mail.channel@neighbor.com>' % frog_groups.id)
+                          extra='In-Reply-To: <12321321-openerp-%d-mail.channel@neighbor.com>' % self.group_public.id)
 
         # Test created messages
-        self.assertEqual(len(self.group_public.message_ids), 2, 'message_process: group should contain 6 messages')
-        self.assertEqual(len(self.group_public.message_ids[0].child_ids), 0, 'message_process: msg1 should not have children')
+        self.assertEqual(len(self.group_public.message_ids), 1)
+        self.assertEqual(len(self.group_public.message_ids[0].child_ids), 0)
 
     @mute_logger('odoo.addons.mail.models.mail_thread', 'odoo.models')
     def test_message_process_duplicate(self):