The information on this page is likely to be outdated, since I no longer deal with deploying and operating large-scale messaging systems.
Microsoft's mail and collaboration plaftorm, which evolved from an X.400-like MTA into an SMTP-optimized behemoth with client-side MAPI, IMAP4, NNTP, POP3 and WebDAV support.
- DAVMail, a combined POP/IMAP/SMTP/Caldav/Carddav/LDAP gateway between Exchange and open protocols.
- Zarafa, another company doing a clone stack.
- OpenChange, an attempt at re-implementing the necessary protocols in a portable fashion, supplying MAPI libraries, Wireshark dissectors, the works.