Sendmail is an open source Mail Transfer Agent (MTA). It is a computer program that delivers mail from one machine to another.

Originally, Sendmail was written by Eric Allman in 1980s. In 1983, Sendmail was first shipped with BSD 4.1, the first BSD version that includes TCP/IP. The current version of Sendmail is 8.13.4, which has been released on March 27, 2005.
Sendmail implements a general inter-network mail routing facility, featuring aliasing and forwarding, automatic routing to network gateways, and flexible configuration.

Sendmail is a flexible program, which supports a wide range of mail transfer and delivery systems including the extremely popular SMTP.

Though Sendmail has been criticized as slow and overcomplicated MTA, it is the most popualar among the MTAs on the Internet. According to a report of November 2001, approximately 42% mail servers on the Internet were using Sendmail, which was indeed a great achievement in this tremendously competitive web world. However, three serious vulnerabilities were detected in Sendmail in 2003.
Sendmail is usually run as superuser, to highlight a severe security threat if compromised.

Sendmail X is said to be the next generation of Sendmail. It is under development and the design resembles with the design of Postfix

Sendmail X.0 is intended to be used as a secure and efficient mail gateway. So far, it does not provide any mail content modification capabilities, e.g., address masquerading.