To: field holding our doain, not the sender's if they use mailing list

[Guest_imported]

We are receiving emails from outside our domain but when the sender puts a 'reply to:' field in, which is a mailing list, the 'reply to:' field get its domain changed to give the impression that the sender is in our domain. I need to retain the name and correct domain of the 'reply to:' field as specificed by the sender.

so :-

sender = john@dawg.com
reply to = mail_list@dawg.com

recipient = chest@myworld.com

The header for this email says :-

---------------------------------------------------
From:john@dawg.com[mailto:john@dawg.com]
Sent:05 May 2001 18.38
To:mail_list@myworld.com
Subject: supercalifrag...
etc
etc
----------------------------------------------------

The To: field *should* say mail_list@dawg.com...

I think the sendmail is receiving this without the intended domain and then adding ours on, which is wrong. if it cannot expand/correct the email, I'd rather it scrapped it or lets root/postmaster know...?