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, and has spread both as free software and proprietary software.
Allman had written the original ARPANET delivermail which shipped in 1979 with 4.0 and 4.1 BSD. He wrote Sendmail as a derivative of delivermail early in the 1980s at UC Berkeley. It shipped with BSD 4.1c in 1983, the first BSD version that included TCP/IP protocols.
In 2001, approximately 42% of the publicly-reachable mail-servers on the Internet ran 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 total.
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 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.
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