This is my situation:

I own the domain "example.co.za" hosted by others. My fedora box, fetches user mail with fetchmail from the "others", and sendmail / mailscanner send mail out via "smart host"

What i want to achieve is when outbound mail meets certain criteria (i.e. there is a corresponding local user) it should be delivered locally. (To save wasting ADSL cap on mails bouncing around)

"Mobile" users who need access to mail on the road collect and send and receive there mail from the external domain hosting server, not my internal server.

When mail is sent to "Mobile" users from a local user , this mail must NOT get delivered to the local user (which does exist, all my mail users have user accounts on the server for other file access) but should go out via sendmail and the smarthost to the "other" server, thus being accessible to them on the road.

My mail server is not visible to the outside world and i dont particularly want to go that route.

I have played around with virtusers but so far have not achieved my goal.

Anyone who can assist? Thanks