sendmail

Sendmail is a mail transfer agent (MTA) for Unix-like systems that routes and delivers email between hosts. It implements the Simple Mail Transfer Protocol (SMTP) and is designed to handle large volumes of mail with configurable routing, queuing, and policy control. For decades it was one of the most widely deployed MTAs in the Internet.

History and development

Sendmail was developed by Eric Allman at the University of California, Berkeley, with early releases in the 1980s. It evolved into the Sendmail 8 family, which became the dominant MTA on many Unix systems for many years. In the 2000s, other MTAs such as Postfix and Exim gained popularity, and Sendmail’s market share declined on new deployments. It remains in use today, particularly on legacy systems or environments that require its extensive configurability.

Architecture and configuration

Sendmail consists of a daemon that controls worker processes and a queue manager responsible for message delivery retries. Configuration is traditionally written in sendmail.mc and processed by the m4 macro processor to produce the sendmail.cf file loaded at startup. The MTA supports SMTP for transport, DNS-based routing via MX records, and a wide range of rewriting and policy rules. It provides aliasing through virtusertable and a flexible policy framework, and can be extended with the Milter interface for mail filtering. Security features include support for TLS/STARTTLS and SASL authentication for outgoing mail.

License and usage

Historically, Sendmail was distributed under a permissive BSD-like license with variants over time. Current releases are available as open-source software and are used in environments that value its configurability and historical integration with diverse Unix systems.