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.