Courier IMAP is described as 'The Courier IMAP server is a fast, scalable, enterprise IMAP server that uses Maildirs. Many E-mail service providers use the Courier IMAP server to easy handle hundreds of thousands of mail accounts' and is an app in the Office & Productivity category. There are four alternatives to Courier IMAP for Linux, Mac and BSD. The best alternative is Dovecot, which is both free and Open Source. Other great apps like Courier IMAP are Cyrus IMAP (Free, Open Source), Archiveopteryx (Free, Open Source) and DBMail (Free, Open Source).
The Cyrus IMAP server differs from other IMAP server implementations in that it is generally intended to be run on sealed servers, where normal users cannot log in. Users can access mail through the IMAP/IMAP-S, JMAP, POP3/POP3-S or KPOP protocols.
Archiveopteryx is an Internet mail server, optimised to support long-term archival storage. It seeks to make it practical not only to manage large archives, but to use the information therein on a daily basis instead of relegating it to offline storage.