The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. Individual components can be enabled or disabled at will. Courier now implements basic web-based calendaring and scheduling services integrated in the webmail module. Advanced groupware calendaring services will follow soon.
Courier’s source code should compile on most POSIX-based operating systems based on Linux, and BSD-derived kernels. Courier should also compile on Solaris and AIX, with some help from Sun’s or IBM’s freeware add-on tools for their respective operating systems.
Courier evolved out of several related projects, that merged together (more on that later). Courier implements SMTP extensions for mailing list management and spam filtering. Courier can function as an intermediate mail relay, relaying mail between an internal LAN and the Internet, or perform final delivery to mailboxes. Courier uses maildirs as its native mail storage format, but it can also deliver mail to legacy mailbox files as well. Courier’s configuration is set by plain text files and Perl scripts. Most of Courier’s configuration can now be adjusted from a web browser, using Courier’s web-based administration module.
Courier can provide mail services for regular operating system accounts. Courier can also provide mail services for virtual mail accounts, managed by an LDAP, MySQL, or PostgreSQL-based authentication database.
Certain portions of Courier - the mail filtering engine, the webmail server and IMAP server - are also available are separate, smaller, packages that can be used with other mail servers.