diff --git a/addons/mail/models/mail_thread.py b/addons/mail/models/mail_thread.py
index 9f76813eac2..9153628b83b 100644
--- a/addons/mail/models/mail_thread.py
+++ b/addons/mail/models/mail_thread.py
@@ -110,7 +110,7 @@ class MailThread(models.AbstractModel):
         'Number of error', compute='_compute_message_has_error',
         help="Number of messages with delivery error")
     message_attachment_count = fields.Integer('Attachment Count', compute='_compute_message_attachment_count')
-    message_main_attachment_id = fields.Many2one(string="Main Attachment", comodel_name='ir.attachment', index=True)
+    message_main_attachment_id = fields.Many2one(string="Main Attachment", comodel_name='ir.attachment', index=True, copy=False)
 
     @api.one
     @api.depends('message_follower_ids')