diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 462ec514378..84be9b714a2 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -2214,6 +2214,8 @@ class MailThread(models.AbstractModel):
             base_template = False
 
         mail_subject = message.subject or (message.record_name and 'Re: %s' % message.record_name) # in cache, no queries
+        # Replace new lines by spaces to conform to email headers requirements
+        mail_subject = ' '.join((mail_subject or '').splitlines())
         # prepare notification mail values
         base_mail_values = {
             'mail_message_id': message.id,
diff --git a/addons/test_mail/tests/test_message_post.py b/addons/test_mail/tests/test_message_post.py
index f1c970881e6..b616b60831d 100644
--- a/addons/test_mail/tests/test_message_post.py
+++ b/addons/test_mail/tests/test_message_post.py
@@ -302,6 +302,16 @@ class TestMessagePost(TestMailCommon, TestRecipients):
         self.assertEqual(new_msg.parent_id.id, parent_msg.id, 'message_post: flatten error')
         self.assertEqual(new_msg.partner_ids, self.env['res.partner'])
 
+    @mute_logger('odoo.addons.mail.models.mail_mail')
+    def test_post_email_with_multiline_subject(self):
+        _body, _body_alt, _subject = '<p>Test Body</p>', 'Test Body', '1st line\n2nd line'
+        msg = self.test_record.with_user(self.user_employee).message_post(
+            body=_body, subject=_subject,
+            message_type='comment', subtype_xmlid='mail.mt_comment',
+            partner_ids=[self.partner_1.id, self.partner_2.id]
+        )
+        self.assertEqual(msg.subject, '1st line 2nd line')
+
     @mute_logger('odoo.addons.mail.models.mail_mail')
     def test_post_portal_ok(self):
         self.test_record.message_subscribe((self.partner_1 | self.user_employee.partner_id).ids)