Postfix

Postfix is a mail transfer agent originally created at IBM research as an alternative to Sendmail and now maintained by its author after a tenure at Google. It is designed to handle email routing and delivery on UNIX‑like operating systems, including AIX, BSD, HP‑UX, Linux, macOS, Solaris and others. The software can be built from source or obtained as pre‑compiled packages from various OS and appliance vendors.

The implementation focuses on speed, ease of administration, and security. While its command‑line interface and configuration files retain a familiar “Sendmail‑ish” style, the underlying architecture is completely rewritten to avoid the complexities and vulnerabilities associated with the original program. Administrators can manage queues, policies and access controls through straightforward configuration directives.

Postfix is released under the IPL‑1.0 license, is self‑hostable, and does not require a subscription or paid tier. It is considered stable and is widely used in production environments that need a reliable, open‑source mail server.