Sendmail is a general purpose internetworkemail 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.
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Allman had written the original ARPANETdelivermail which shipped in 1979 with 4.0 and 4.1 BSD. He wrote Sendmail as a derivative of delivermail in the early 1980s at UC Berkeley. It shipped with BSD 4.1c in 1983, the first BSD version that included TCP/IP protocols.
In 1996, approximately 80% of the publicly reachable mail-servers on the Internet ran Sendmail.[2] More recent surveys have suggested a decline, with 3.64% of mail servers in March 2021 detected as running Sendmail in a study performed by E-Soft, Inc.[3] A previous survey (December 2007 or earlier) reported 24% of mail servers running Sendmail according to a study performed by Mail Radar.[4]
Allman designed Sendmail to incorporate great flexibility, but it can be daunting to configure for novices.[5] 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, DECnet‘s Mail-11, HylaFax, QuickPage and UUCP. Additionally, Sendmail v8.12 as of September 2001[update] introduced support for milters – external mail filtering programs that can participate in each step of the SMTP conversation.
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