Cyrus IMAP is described as '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' and is an app in the Office & Productivity category. There are five alternatives to Cyrus IMAP for Linux, Mac, BSD and Self-Hosted solutions. The best alternative is Dovecot, which is both free and Open Source. Other great apps like Cyrus IMAP are Magma (Free, Open Source), Archiveopteryx (Free, Open Source), Courier IMAP (Free, Open Source) and DBMail (Free, Open Source).
Magma is a fully-featured, commercial-grade email server with support for SMTP, POP3, IMAP, HTTP, and an internal protocol MOLTEN. Support for the DIME ecosystem of advanced encryption protocols is currently in active development. Originally created by Ladar Levison for lavabit.
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.