The best replacement for microsoft exchange is lotus notes.

The best alternatives to microsoft exchange are rated as following:
★★★★★  lotus notes
★★★★  gmail
★★★  google apps
★★  zimbra
★★  microsoft outlook
★★  lotus domino
★★  pop3
★  groupwise
★  mdaemon
★  linux mail server
★  domino
★  open xchange
  gmail iphone
  mobile me
  google mail
  connected to microsoft exchange
  microsoft mail

Your alternative for microsoft exchange is not listed? Suggest!
Speaking about microsoft exchange, one probably shouldn't forget about mac mail.

Other search terms related to microsoft exchange and their ratings are:

★★★★★ mac mail
★★ gmail iphone
You have in mind a companion for microsoft exchange and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of microsoft exchange in all years. Click on graph for more.
microsoft exchange
lotus notes