Exim

Exim is a Unix‑based message transfer agent that routes email between servers and delivers it to local mailboxes. It implements the SMTP protocol and can be configured to handle inbound and outbound mail, perform address rewriting, and apply extensive filtering and authentication checks. The software supports flexible routing rules, allowing administrators to direct messages based on domain, host, or custom criteria.

The package is distributed under the GPL‑3.0 license, making it freely available for self‑hosting without subscription or tier restrictions. It is maintained as a stable, open‑source project, with regular security releases and optional beta builds for testing. Documentation, mailing lists, and a wiki provide guidance for installation and configuration, and the system can serve as a drop‑in replacement for other MTAs such as Sendmail, though its configuration syntax differs.

Exim is intended for system administrators and organizations that need a customizable, Unix‑native mail handling solution. Its extensive routing flexibility and built‑in spam and policy controls make it suitable for a wide range of email infrastructure deployments, from small servers to larger, multi‑domain environments.