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. Greg Olson incorporated the company and drove sales in the early years. When sales went flat in Europe, he took charge of the territory and European sales resumed their brisk pace. He served as Sendmail's CEO for a time.