PostPath (http://www.postpath.com/) is supposed to be a drop-in replacement for Exchange - e.g no MAPI connector needed, they reverse-engineered the Exchange protocol.
However, Cisco bought them recently, and unlike most Cisco acquisitions (which continue on nearly unchanged, e.g Linksys), PostPath seems to have been swallowed up. I hear their intent is to turn PostPath into an email-as-a-service product. Thus, not available as an in-house server any longer. Sigh.
Cisco - make PostPath available for in-house (non-SAAS/cloud) servers again!