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.