The best replacement for exchange 2013 is google apps.

The best alternatives to exchange 2013 are rated as following:
★★★★★  google apps
★★★★  exchange online
★★★  zimbra
★★  zimbra 8
★★  enterprise vault
★  mdaemon
★  office 365

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

Other search terms related to exchange 2013 and their ratings are:
★★★★★ tmg
★★★★★ outlook 2007
★★★★ exchange 2007
★★★ server 2012 r2
★★ lync 2013
★ outlook 2010
★ office 2007
2007
You have in mind a companion for exchange 2013 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of exchange 2013 in all years. Click on graph for more.
exchange 2013
google apps
exchange online