Like its big brother Sendmail, Postfix is an open source mail transfer agent (MTA) for routing and delivery of e-mail. It is fast and easy to administer. You can even use for administration a nice web inteface of Webmin. Postfix was originally written by Wietse Venema and released somewhere in 1999. The majority of our web analytics company MTAs are Sendmail servers, but we also use a couple of Postfix machines for handling humongous amounts of statistical reports that our Web analysts send back and forth.
Since 1999, Postfix definitely matured. Right now, I see two major strong points for using this MTA. Fist of all, it has amazing resilience against buffer overflows. Sendmail is more vulnerable, I must say (although, I love Sendmail).
Second major plus is the way Postfix is handling huge amounts of e-mail. This baby just does not go down, no matter what! Another one is its handling of large amounts of e-mail. Its creator Wietse Venema (also a creator of tcp wrappers) built Postfix as a cooperating network of different daemons. Each daemon fulfills a single task using minimum privileges. Even if a daemon is compromised, the impact remains limited to that daemon. It cannot spread throughout your entire system. There is only one process with root privileges, and a few that actually write to disk or invoke external programs. Besides, most Postfix daemons can be easily chrooted.
There are several nice books on Postfix that you can buy at Amazon.com. One of them is really good. Yet, here is what I say. Save yourself some money. I suggest you go straight to Postfix online documentation and get everything that is written in the books for free. I am not kidding, it is true. Click on the link above and check it out yourself. If you think, that this is too complicated, choose from here Postfix basic configuration online tutorial.