diff --git a/addons/mail/mail_thread.py b/addons/mail/mail_thread.py
index a6060f5f54f..f7a89177e35 100644
--- a/addons/mail/mail_thread.py
+++ b/addons/mail/mail_thread.py
@@ -967,12 +967,15 @@ class mail_thread(osv.AbstractModel):
                                     ], limit=1, context=context)
                 if author_ids:
                     kwargs['author_id'] = author_ids[0]
+        author_id = kwargs.get('author_id')
+        if author_id is None:  # keep False values
+            author_id = self.pool.get('mail.message')._get_default_author(cr, uid, context=context)
 
         # 1: Handle content subtype: if plaintext, converto into HTML
         if content_subtype == 'plaintext':
             body = tools.plaintext2html(body)
 
-        # 2: Private message: add recipients (recipients and author of parent message)
+        # 2: Private message: add recipients (recipients and author of parent message) - current author
         #   + legacy-code management (! we manage only 4 and 6 commands)
         partner_ids = set()
         kwargs_partner_ids = kwargs.pop('partner_ids', [])
@@ -985,12 +988,13 @@ class mail_thread(osv.AbstractModel):
                 partner_ids.add(partner_id)
             else:
                 pass  # we do not manage anything else
-
         if parent_id and not model:
             parent_message = mail_message.browse(cr, uid, parent_id, context=context)
-            partner_ids |= set([partner.id for partner in parent_message.partner_ids])
+            private_followers = set([partner.id for partner in parent_message.partner_ids])
             if parent_message.author_id:
-                partner_ids.add(parent_message.author_id.id)
+                private_followers.add(parent_message.author_id.id)
+            private_followers -= set([author_id])
+            partner_ids |= private_followers
 
         # 3. Attachments
         #   - HACK TDE FIXME: Chatter: attachments linked to the document (not done JS-side), load the message
@@ -1051,6 +1055,7 @@ class mail_thread(osv.AbstractModel):
 
         values = kwargs
         values.update({
+            'author_id': author_id,
             'model': model,
             'res_id': thread_id or False,
             'body': body,
diff --git a/addons/mail/tests/test_mail_gateway.py b/addons/mail/tests/test_mail_gateway.py
index 60a08f0c62f..a68011298c7 100644
--- a/addons/mail/tests/test_mail_gateway.py
+++ b/addons/mail/tests/test_mail_gateway.py
@@ -372,4 +372,55 @@ class TestMailgateway(TestMailBase):
 
     def test_20_private_discussion(self):
         """ Testing private discussion between partners. """
-        pass
+        cr, uid = self.cr, self.uid
+
+        # Do: Raoul writes to Bert and Administrator, with a thread_model in context that should not be taken into account
+        msg1_pids = [self.partner_admin_id, self.partner_bert_id]
+        msg1_id = self.mail_thread.message_post(cr, self.user_raoul_id, False,
+                        partner_ids=msg1_pids,
+                        subtype='mail.mt_comment',
+                        context={'thread_model': 'mail.group'})
+
+        # Test: message recipients
+        msg = self.mail_message.browse(cr, uid, msg1_id)
+        msg_pids = [p.id for p in msg.partner_ids]
+        msg_nids = [p.id for p in msg.notified_partner_ids]
+        test_pids = msg1_pids
+        test_nids = msg1_pids
+        self.assertEqual(set(msg_pids), set(test_pids),
+                        'message_post: private discussion: incorrect recipients')
+        self.assertEqual(set(msg_nids), set(test_nids),
+                        'message_post: private discussion: incorrect notified recipients')
+        self.assertEqual(msg.model, False,
+                        'message_post: private discussion: context key "thread_model" not correctly ignored when having no res_id')
+
+        # Do: Bert replies through mailgateway (is a customer)
+        msg2_id = self.mail_thread.message_post(cr, uid, False,
+                        author_id=self.partner_bert_id,
+                        parent_id=msg1_id, subtype='mail.mt_comment')
+
+        # Test: message recipients
+        msg = self.mail_message.browse(cr, uid, msg2_id)
+        msg_pids = [p.id for p in msg.partner_ids]
+        msg_nids = [p.id for p in msg.notified_partner_ids]
+        test_pids = [self.partner_admin_id, self.partner_raoul_id]
+        test_nids = test_pids
+        self.assertEqual(set(msg_pids), set(test_pids),
+                        'message_post: private discussion: incorrect recipients when replying')
+        self.assertEqual(set(msg_nids), set(test_nids),
+                        'message_post: private discussion: incorrect notified recipients when replying')
+
+        # Do: Administrator replies
+        msg3_id = self.mail_thread.message_post(cr, uid, False,
+                        parent_id=msg2_id, subtype='mail.mt_comment')
+
+        # Test: message recipients
+        msg = self.mail_message.browse(cr, uid, msg3_id)
+        msg_pids = [p.id for p in msg.partner_ids]
+        msg_nids = [p.id for p in msg.notified_partner_ids]
+        test_pids = [self.partner_bert_id, self.partner_raoul_id]
+        test_nids = test_pids
+        self.assertEqual(set(msg_pids), set(test_pids),
+                        'message_post: private discussion: incorrect recipients when replying')
+        self.assertEqual(set(msg_nids), set(test_nids),
+                        'message_post: private discussion: incorrect notified recipients when replying')