The best replacement for zimbra is exchange.

The best alternatives to zimbra are rated as following:
★★★★★  exchange
★★★★  thunderbird
★★★  google apps
★★★  microsoft exchange server
★★★  ms exchange
★★  scalix
★  zarafa
★  exchange 2013
★  exchange 2010
★  roundcube
★  citadel
★  atmail
★  iredmail
★  em client
★  postfix
★  kerio
  horde
  open xchange
  communigate
  kolab
  squirrelmail
  groupwise
  funambol
  open-xchange

Your alternative for zimbra is not listed? Suggest!
Speaking about zimbra, one probably shouldn't forget about outlook 2013.

Other search terms related to zimbra and their ratings are:
★★★★★ outlook 2013
★★ rda
You have in mind a companion for zimbra and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of zimbra in all years. Click on graph for more.
zimbra
exchange
thunderbird