Argosoft Mail Server:
[link|http://www.webattack.com/get/argomail.shtml|http://www.webattack...et/argomail.shtml]
I've used it and it seems to work fine once you get the relay settings set right.

Mail Enable:
[link|http://www.webattack.com/get/mailenable.shtml|http://www.webattack.../mailenable.shtml]
I use this one, it seems to be easy to use and runs as an NT/W2K/XP service. It also does mailing lists as well.

Mailtraqw is one I haven't tried yet:
[link|http://www.mailtraq.com/|http://www.mailtraq.com/]
I heard some good things about it.

Good luck, pick the one that works best for you.