SENDMAIL Mail Server
Sendmail is the most popular mail transfer agent (MTA) on the Internet. Its popularity is due in part to its position as the standard MTA under most variants of the Unix and other Unix-like operating system.
Sendmail is designed with great flexibility, but it can be daunting to configure for novices. Standard configuration packages delivered with the source code distribution require the use of the M4 macro language which hides much of the configuration complexity. The configuration defines the site-local mail delivery options and their access parameters, the mechanism of forwarding mail to remote sites, as well as many application tuning parameters.
Sendmail supports a variety of mail transfer protocols, including SMTP, ESMTP, DECnet's mail11, HylaFax, QuickPage and UUCP. Additionally, Sendmail v8.12 as of September 2001 introduced support for milters - external mail filtering programs that can participate in each step of the SMTP conversation.