Mail-Delivery-Agents
Mail Delivery Agents (MDAs) - IMAP/POP3 server software.
3 Apps
Dovecot
dovecot.org
IMAP and POP3 server written primarily with security in mind.
D DavMail
davmail.sourceforge.net
POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway allowing users to use any mail/calendar client with an Exchange server, even from the internet or behind a firewall through Outlook Web Access.
C
Cyrus IMAP
cyrusimap.org
Email (IMAP/POP3), contacts and calendar server.