Qmail is an SMTP server, so port 25 is enough, but I'd seriously consider extra services on your box, like spam filtering, antivirus, you may even want to go down the path of SMTP over ssl.
I've used qmail for years and love it. check out
QmailRocks.org
although the QMR website is a little out of date these days, its still a great guide. I have used it on Fedora 5 and Debian 3.1 and 4.0 even though those guides are written for versions many releases prior.
