check the message mime
view → show → page source
and if you can recreate, maybe try and trap the message at the router
(open mail.box when router task is temporarily turned off)
and compare the routed page source with the trapped message’s
Check the From field in doc properties to see if it contains the sender’s address and that the SMTPOriginator field is that same.
If they are correct, then I’d replace the design of the mail template on the user’s mailfile. If you’ve got Designer, check out the design beforehand to see if the Memo Form has been changed or corrupted.