Dovecot is an open source IMAP and POP3 server for Linux/UNIX-like systems, written with security primarily in mind.
Elm Millennium Edition is an interactive mail system based on Elm 2.4.
AXIGEN – a carrier class messaging solution backed up by first rate tech support.
The Cyrus IMAP server is generally intended to be run on sealed systems, where normal users are not permitted to log in.
IMAP::Admin is a Perl module for basic IMAP server administration.
Courier-IMAP is a fast, scalable, enterprise IMAP server that uses Maildirs.
A standards-based webmail package written in PHP.