Dovecot is an open source IMAP and POP3 email server for Linux/UNIX-like
systems, written with security primarily in mind. Dovecot is an excellent
choice for both small and large installations. It's fast, simple to set up,
requires no special administration and it uses very little memory.

Dovecot is high-performing, self-optimizing, self-healing, and easily
extensible. It includes IMAP4rev1 and POP3 support. IPv6, SSL and TLS are
supported. It supports multiple commonly used IMAP extensions, including SORT,
THREAD and IDLE.

WWW: http://www.dovecot.org/