I don't want to auto-magically configure mail clients to display user@mail.com in from field
If you dont, you will have issues with the mail being rejected by other servers on the basis it is an unresolveable domain.
However, i want fetchmail to replace headers if it detects that mail is going out of the local domain.
I dont believe the fetchmail contrib will re-write headers (nor should it).
If I understand you correctly, this is in fact a seperate issue. Fetchmail is about collecting mail and redistributing it whereas your issue is about dispatching mail.
I think that's how it goes with SBS2008 and it's Exchange
I dont know POPConn but that is not quite how exchange handles it.
As you have already been advised, this is a mail client configuration issue however it does have SMTP implications since the SMTP server needs to be able to handle the relay somehow either by way of a virtual domain or a user authenntication or some other method.
Good Luck - email relays can be very challenging.