Postfix is Wietse Venema's attempt to provide 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.
This software was formerly known as VMailer. It was released by the end of 1998 as the IBM Secure Mailer. From then on it has lived on as Postfix.