Sendmail is a general purpose internetwork email routing facility that supports many kinds of mail-transfer and delivery methods, including the Simple Mail Transfer Protocol (SMTP) used for email transport over the Internet.
A descendant of the delivermail program written by Eric Allman, Sendmail is a well-known project of the free and open source software and Unix communities. It has spread both as free software and proprietary software.
In 1996, approximately 80% of the publicly reachable mail-servers on the Internet ran Sendmail. More recent surveys have suggested a decline, with 4.67% of mail servers in October 2017 detected as running Sendmail in a study performed by E-Soft, Inc. Other surveys have suggested a slight decrease, with 24% of mail servers in August 2015 detected as running Sendmail in a study performed by Mail Radar.
Allman designed Sendmail to incorporate 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 Mail-11, HylaFax, QuickPage and UUCP. Additionally, Sendmail v8.12 introduced support for - external mail filtering programs that can participate in each step of the SMTP conversation.
The information derives from ftp://ftp.sendmail.org/pub/sendmail/RELEASE_NOTES from sendmail distribution.
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 UNIX-HATERS Handbook dedicated an entire chapter to perceived problems and weaknesses of sendmail.
— the sendmail message submission program.