Postfix is a lightweight MTA (unlike the old, bloated SendMail) written from the ground up to be fast and secure.
It was written by Weitse Venema. He did some important stuff for Internet security in *nix applications, but cant remember what exactly. Even still he is highly regarded in the security field and his implementation of an MTA is widely adopted in both personal and commerical environments.
It is configured using a format similar to sendmail ("sendmail-ish") which makes it easier to migrate from sendmail to Postfix.
