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.

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.

[6] Sendmail originated in the early days of the Internet, an era when considerations of security did not play a primary role in the development of network software.

Sendmail itself incorporated a certain amount of privilege separation in order to avoid exposure to security issues.