Postfix is a lightweight MTA (unlike the old, bloated SendMail) written from the ground up to be fast and secure, and designed to be a drop-in replacement for SendMail. It is popular 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. Its design is modular, ie. it is made up of several smaller applications, each of which is designed to do one task only (e.g. messages are received via SMTP by one program, are delivered locally by another, and are delivered to another host via SMTP by yet another seperate program). This approach allows individual processes to be replaced to meet the user's needs, which is useful in large enterprise and ISP environments where custom solutions are required. It also means means that the system has lower resource overhead than a monolithic server.