Freegate makes a nice little box that does ISDN and modem dialup and also had a built in mail server. The mail server can do email via a POP domain.
In order to make this work, you have to set a header setting with envelope recepient information. That header is automatically added to the emails by NTmail when you setup to do a pop domain. The header is
X-Originally-To
It's really not an NT tip, but I didn't want to lose it as it took me days to find it the first time.
Geo.