Postfix Mail Transport Agent (MTA)
Postfix is a Mail Transport Agent (MTA). It is not a complete mail system, but focuses on the MTA component, and does not include a Mail Delivery Agent (MDA), such as IMAP support, nor does it include a Mail User Agent (MUA), such as a web mail client. Other popular MTA-focused apps include Exim and Sendmail.
The advantages of Postfix in terms of flexibility, stability, and security are generally offset by the complexity of configuration.