Citadel is a highly integrated Groupware Platform with a Web 2.0 enabled Web interface, but also providing SMTP, IMAP, POP3 and GroupDAV access to its content.

Citadel offers versatile email services with very low administration needed. It provides its own implementations of these server protocols:
IMAP, POP3, SMTP, ManageSieve, XMPP, Citadel.