Postfix is a Mail Transfer Agent, and can be considered a replacement for Sendmail, qmail, Exim, and others. It is most often found running the public facing mail gateways of ISPs and large corporations.
When Wietse Venema needed a fast, secure MTA, he decided that rather than attempt to streamline an existing program, or retrofit security features, it would be better to just start from scratch. Postfix, (or VMailer as it was originally known) was born in 1997 and has enjoyed a strong following ever since.
We recommend Postfix for busy and exposed mail gateways, a corporate perimeter Internet MTA. In such deployments we typically install it on minimalist FreeBSD or Solaris systems.
How long have you got? Postfix can be installed and configured very quickly, particularly for sites currently dependent on Sendmail, with which it shares good deal of compatibility. What's more, because everything in Postfix is a table, it scales from home systems up to an ISP sized mail relays. Tables can be accessed via flat text files to full relational databases.
Lehman Brothers and their 23000 staff, Email is a mission critical service. Postini, the Email security service provider, and ourselves at 360is.