Sendmail and Qmail are pretty much standard over the Internet. There are others.
Sendmail is pretty complex to setup, but has enormous flexibility. However, if it is not setup correctly, it can be a source of extreme headaches.
Qmail is simpler to setup and has a similar feature set to Sendmail but is probably more bulletproof to configure.
Both scale well (if you know what you are doing). Both are native to UNIX. If you are running a Windows server-wannabe thing, then either of the above is probably not suitable.
