When I last did it, I just used what information was already there in the guide.
http://netbsd.org/guide/en/chap-mail.html
Then for the spam and virus, I just used a random Linux/postfix howto I found by googling on the web. It was very easy. The guide will tell you anything specific you need to know or remember. After that, any *nix guide for setting up postfix with spam protection will work for you.
How do make NetBSD mailserver?
