diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index b5f83f14deb..2c0c57cac51 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -1702,6 +1702,8 @@ class MailThread(models.AbstractModel):
                     continue
                 if isinstance(content, str):
                     content = content.encode('utf-8')
+                elif isinstance(content, EmailMessage):
+                    content = content.as_bytes()
                 elif content is None:
                     continue
                 attachement_values= {