A descendant of delivermail program written by Eric Allman, Sendmail is a well-known project of the free and open source software and Unix communities, and is distributed both as free software and proprietary software.
Allman had written the original ARPANET delivermail which was shipped in 1979 with 4.0 and 4.1 BSD. Sendmail, a derivative, was written in the early 1980s at UC Berkeley, and shipped with BSD 4.1c in 1983 (the first BSD version to include TCP/IP).
Sendmail remains the most popular MTA on the Internet. Its popularity is due in part to its position as the standard MTA under most variants of the Unix operating system. According to latest D. J. Bernstein SMTP survey , as of November 2001 approximately 42% of the publicly reachable mail servers on the Internet were running Sendmail. More recent surveys have suggested a decline, with 29.4% of mail servers in August 2007 detected as running Sendmail in a study performed by E-Soft, Inc. Sendmail is trailed by Microsoft Exchange Server, Exim, and Postfix, these four being the only mail servers with more than 10% of the installed base.
Sendmail can be difficult to configure. Whereas nearly all popular Unix based server software has human readable configuration files, Sendmail's configuration is not considered human readable. Instead, the Sendmail authors recommend that administrators learn and use macro language tools, particularly M4 to configure Sendmail. Sendmail is unique amongst Unix based MTAs in this requirement, and no MTA developed since Sendmail requires or recommends the use of macro languages for configuration.
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.
Sendmail itself incorporated a certain amount of privilege separation in order to avoid exposure to security issues. Current versions of Sendmail, like other modern MTAs, incorporate a number of security improvements and optional features that can be configured to improve security and help prevent abuse.
The information is based on RELEASE_NOTES file from sendmail distribution.