Courier Mail Server (Courier MTA)

Courier Mail Server (often referred to as Courier MTA) is an integrated email server suite that combines mail transfer, mail access, and webmail components under a single framework. It supports common Internet mail protocols and can be used as a relay, a final-delivery server for multiple domains, or a mixed setup.

Key Features

• ESMTP mail transfer agent with DSN, PIPELINING, 8BITMIME, and message submission support
• Integrated IMAP and POP3 servers (maildir-based mail storage)
• Integrated webmail (SqWebMail) with basic web-based calendaring and scheduling
• Virtual mail accounts backed by LDAP, MySQL, or PostgreSQL, as well as system accounts
• Built-in mail filtering with support for external filters and per-user filtering rules via Maildrop
• Mailing list manager with automated bounce processing
• TLS/SSL support for SMTP, IMAP, POP3, and webmail (STARTTLS and certificate validation options)
• Optional features such as DNS blacklist checks, authenticated SMTP, and rate limiting

Use Cases

• Running a combined SMTP/IMAP/POP3/webmail stack for a small organization or homelab
• Hosting multi-domain email with virtual users stored in LDAP or SQL databases
• Operating an intermediate relay with integrated filtering and policy controls

Limitations and Considerations

• IMAP/POP3/webmail components are designed primarily for maildir; mbox support is limited
• Some SMTP features (such as ETRN) are intentionally not supported

Courier is best suited for administrators who want a cohesive mail system with integrated access services and filtering. Its modular components can be enabled or disabled as needed, and several parts are also available as standalone packages for use with other mail systems.