Hi Have you tried using a SMTP Smarthost?
Normally you can send all mail to your domain host / internet service provider…. you may have to give them your IP so they can allow you to relay through there smarthost server..
OK I did a liitle search and come up with this link
It should help… if your service provider will allow you to use their smarthost by just giving them your external IP you dont need to worry about Authentication… otherwise you will have to get them to give you username / password and follow the instructions in the link.
Hope it helps