The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, TLS, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework.

The web-based administration and configuration tool for the Courier mail server is capable of changing the settings of the MTA, IMAP, POP and Webmail servers and the LDAP, MySQL and PostgreSQL authentication modules. Only the installed parts of the Courier mail server show up in the administration frontend.