Postfix is arguably one of the fastest and most secure email MTAs (Message Transfer Agent) available. It is also simple to configure in comparison to sendmail.
What is Postfix? It is Wietse Venema's mailer that started life as an alternative to the widely-used Sendmail program.
Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Thus, the outside has a sendmail-ish flavor, but the inside is completely different.
The postfix site is available here.
There is a very good and extensive guide on how to set up a small emailserver using postfix on the gentoo-wiki.
There is also more discussion of email server related questions on this site here
Engarde Secure Linux is a distro that can be installed as a postfix email server. (it also can be installed as a web/onlinestore server) It has a web interface so it is easy for a new user to set up and get running. It is designed with security in mind. I have used the free version for about three years and it has been reliable. Free Version. For those that want to buy a system with support and some extra features here is the payware version. Payware Version I did find that after about a year I stopped using the web gui as it was slowing me down and sometimes did not have options that set things exactly the way I wanted them set. It was nice to have however for initial set up when I did not know much about postfix.