Bouncing emails back to sender?

We have the following situation:
girlfriend@mydomain.com sends an email to remotelover@otherhost.com via smtp using the mydomain.com server.
girlfriend@mydomain.com sets the return-to address to girlfriend@mydomain.com, but if the mail deliverly for remotelover@otherhost.com fails, the email gets bounced back to default@mydomain.com - you get the implications.
Is there any way to get around this in exim so that valid bouncing messages are delivered to the original sender instead of the default account?

 

Yes, it is set. The problem doesn't seem to be the catchall for incoming emails though.
The problem seems to be that outgoing mails (sent from any email client) are flagged with the returnpath of
mainusername@hostname.servername.com instead of username@domain.com

 

No, the mails in questions are being sent from any mail client. The user who reported it experienced it with MS Outlook and MS Outlook Express. I tried it on Evolution with the same effect.