More than a year ago an issue came up through the Symantec Support ranks relating to how customers Exchange 2010 mailbox databases were still growing, even after aggressive archiving and clearing out the archived data. Many, many people put a lot of effort in to testing, understanding, debugging and troubleshooting what was happening in this situation, and it was a good deal of work between Symantec and Microsoft.
The result was a blog post by Karl Woodrow:
http://www.symantec.com/connect/blogs/exchange-2010-whitespace-reclamation
That referenced a fix:
http://support.microsoft.com/kb/2621266
And ... now.. Microsoft have released the update, and their own blog post:
http://blogs.technet.com/b/exchange/archive/2012/08/27/exchange-stubbing-and-database-space-reclamat...
There is also a Symantec technote:
http://www.symantec.com/docs/TECH164949
