Has anyone seriously looked at an alternative to Microsoft Exchange?

Paying these CAL licensing requirements over and over for new Exchange versions when they come out is crazy. 2003 CALS are not valid for 2007, 2007 are not valid for 2010, 2010 are not valid for 2013 - Then you also have Server CALS that are required. I'm a Windows geek like most of us but starting to get cranky about all these update costs to the point I'm seriously starting to look into something else.

Not seeing anything that is 100% free that comes anywhere close to Exchange. One or two that are similar to Exchange but not cheap either. If it was a one time cost and a reasonable yearly support cost maybe but would really like to get away from the $10 - $20K + CALS for Exchange every time you want to update the Server.



Zarafa
http://www.zarafa.com/
http://en.wikipedia.org/wiki/Zarafa_%28software%29

Price is actually not very competitive compared to Exchange
Licensing for 500 users has a yearly cost of about $14,000
(Microsoft CALS for 500 users is only about $8,500, and cost of Enterprise for $2,100 or more if you scale up with DAG)

Zarafa needs Cluster aware Linux to scale up, which at that level unless you're a real die-hard Linux person is probably best done with RedHat which also has yearly costs.
I just don't see the cost savings to go from Microsoft to Linux/Zarafa (should either be an extensive costs savings or extensive better software offering - neither of which appear to be there)

ClearOS Professional (Not sure it really scales up or is close to Exchange as most replies are from Home users or Tech not using the mail)

http://ns2.centralpointe.com/index.php
http://ns2.centralpointe.com/Software/clearos-professional-overview.html

No real business replies where someone is using the email and calendaring portion of the system
http://community.spiceworks.com/product/49729-clear-foundation-clearos?source=Selector_73


Possibly this one might be one of the better offerings but still not sure.
http://www.sogo.nu/english.html


Even this one if it it's not a yearly renewal fee - one time cost is okay and reasonable yearly support cost but if they want client renewals each year then it too is a useless switch much like Zarafa as I view it.

http://www.icewarp.com/migration/exchange/