I am using Nathan Fowler's Perl code (referenced above) and it works great. I have people in serveral locations, and one roaming user on a Sprint Palm Phone, and it works for them all.
If you are just looking to relay from your local network or another static network, you can tell smtpfwd to relay from those ip ranges. I belive you can do it through the web interface, and I have done it by editing the templates manually. Search the forums for "relay" for more info.