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 courierfax module implements a faxmail gateway, which faxes a printed copy of an email message to the phone number specified in the email address (e.g. 5558888@fax).