diff --git a/addons/mail/models/mail_activity.py b/addons/mail/models/mail_activity.py
index 63a86c7bbdb..5e5741cf6e6 100644
--- a/addons/mail/models/mail_activity.py
+++ b/addons/mail/models/mail_activity.py
@@ -557,7 +557,7 @@ class MailActivity(models.Model):
                     'res_model': activity_message._name,
                 })
                 activity_message.attachment_ids = message_attachments
-            messages |= activity_message
+            messages += activity_message
 
         next_activities = self.env['mail.activity'].create(next_activities_values)
         self.unlink()  # will unlink activity, dont access `self` after that
diff --git a/addons/mail/models/mail_activity_mixin.py b/addons/mail/models/mail_activity_mixin.py
index 9528886ace5..d29c482b7fc 100644
--- a/addons/mail/models/mail_activity_mixin.py
+++ b/addons/mail/models/mail_activity_mixin.py
@@ -443,7 +443,7 @@ class MailActivityMixin(models.AbstractModel):
         for record in self:
             render_context['object'] = record
             note = self.env['ir.qweb']._render(view_ref, render_context, minimal_qcontext=True, raise_if_not_found=False)
-            activities |= record.activity_schedule(act_type_xmlid=act_type_xmlid, date_deadline=date_deadline, summary=summary, note=note, **act_values)
+            activities += record.activity_schedule(act_type_xmlid=act_type_xmlid, date_deadline=date_deadline, summary=summary, note=note, **act_values)
         return activities
 
     def activity_reschedule(self, act_type_xmlids, user_id=None, date_deadline=None, new_user_id=None):
diff --git a/addons/mail/models/mail_message.py b/addons/mail/models/mail_message.py
index a8bb8e81e6c..617b22308d8 100644
--- a/addons/mail/models/mail_message.py
+++ b/addons/mail/models/mail_message.py
@@ -997,7 +997,7 @@ class Message(models.Model):
                 except AccessError:
                     continue
                 else:
-                    messages |= message
+                    messages += message
         messages_per_partner = defaultdict(lambda: self.env['mail.message'])
         for message in messages:
             if not self.env.user._is_public():
diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index efdeae20be8..43984c8b2df 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -2045,7 +2045,7 @@ class MailThread(models.AbstractModel):
 
     def message_post_with_view(self, views_or_xmlid, **kwargs):
         """ Helper method to send a mail / post a message using a view_id """
-        self._message_compose_with_view(views_or_xmlid, **kwargs)
+        return self._message_compose_with_view(views_or_xmlid, **kwargs)
 
     def message_post_with_template(self, template_id, email_layout_xmlid=None, auto_commit=False, **kwargs):
         """ Helper method to send a mail with a template
@@ -2518,7 +2518,7 @@ class MailThread(models.AbstractModel):
                         'is_read': True,  # discard Inbox notification
                         'notification_status': 'ready',
                     } for recipient_id in tocreate_recipient_ids]
-                emails |= new_email
+                emails += new_email
 
         if notif_create_values:
             SafeNotification.create(notif_create_values)
diff --git a/addons/mail/wizard/mail_compose_message.py b/addons/mail/wizard/mail_compose_message.py
index 4229b0f0c0c..2be52e4ac1d 100644
--- a/addons/mail/wizard/mail_compose_message.py
+++ b/addons/mail/wizard/mail_compose_message.py
@@ -240,7 +240,13 @@ class MailComposer(models.TransientModel):
 
     def _action_send_mail(self, auto_commit=False):
         """ Process the wizard content and proceed with sending the related
-            email(s), rendering any template patterns on the fly if needed. """
+            email(s), rendering any template patterns on the fly if needed.
+
+        :return tuple: (
+            result_mails_su: in mass mode, sent emails (as sudo),
+            result_messages: in comment mode, posted messages
+        )
+        """
         # Several custom layouts make use of the model description at rendering, e.g. in the
         # 'View <document>' button. Some models are used for different business concepts, such as
         # 'purchase.order' which is used for a RFQ and and PO. To avoid confusion, we must use a
@@ -248,6 +254,7 @@ class MailComposer(models.TransientModel):
         # Therefore, we can set the description in the context from the beginning to avoid falling
         # back on the regular display_name retrieved in ``_notify_by_email_prepare_rendering_context()``.
         model_description = self._context.get('model_description')
+        result_mails_su, result_messages = self.env['mail.mail'].sudo(), self.env['mail.message']
 
         for wizard in self:
             # Duplicate attachments linked to the email.template.
@@ -299,7 +306,7 @@ class MailComposer(models.TransientModel):
                 all_mail_values = wizard.get_mail_values(res_ids)
                 for res_id, mail_values in all_mail_values.items():
                     if wizard.composition_mode == 'mass_mail':
-                        batch_mails_sudo |= self.env['mail.mail'].sudo().create(mail_values)
+                        batch_mails_sudo += self.env['mail.mail'].sudo().create(mail_values)
                     else:
                         post_params = dict(
                             message_type=wizard.message_type,
@@ -317,11 +324,14 @@ class MailComposer(models.TransientModel):
                                 # if message_notify returns an empty record set, no recipients where found.
                                 raise UserError(_("No recipient found."))
                         else:
-                            ActiveModel.browse(res_id).message_post(**post_params)
+                            result_messages += ActiveModel.browse(res_id).message_post(**post_params)
 
+                result_mails_su += batch_mails_sudo
                 if wizard.composition_mode == 'mass_mail':
                     batch_mails_sudo.send(auto_commit=auto_commit)
 
+        return result_mails_su, result_messages
+
     def action_save_as_template(self):
         """ hit save as template button: current form value will be a new
             template attached to the current document. """