Courier MTA

Courier MTA is an integrated mail and groupware server that implements standard protocols such as ESMTP, IMAP, POP3, LDAP, SSL and HTTP. It can operate as a final delivery server, an intermediate relay, or a combination of both, handling multiple domains and supporting both system and virtual mail accounts backed by LDAP, MySQL or PostgreSQL. The server stores messages in Maildir format by default but also supports legacy mailbox files, and its configuration is managed through plain‑text files, Perl scripts, and a web‑based administration interface.

The package includes webmail, mailing‑list management, and basic web‑based calendaring and scheduling services, with additional ESMTP extensions for spam filtering and list handling. Individual components—such as the mail filtering engine, webmail, or IMAP server—can be installed separately and used with other mail systems. Features like load limiting, automatic restarts, smart queue organization, and per‑IP vanity configurations aid scalability and reliability.

Courier MTA is self‑hostable, free‑tier, and released under GPL‑3.0, making it a fully open‑source solution for enterprises that require a configurable, standards‑compliant mail infrastructure without subscription fees.