(UPDATE: This is resolved by Exchange RU9)
Do you still have have Exchange 2007 and 2010 coexisting in the same environment and after you installed 2010 come users couldn’t access OWA anymore.?
Your getting this error:
“The mailbox you’re trying to access isn’t currently available. If the problem continues, contact your helpdesk”
AND in the application event log you have and Event ID 46 source MSExchange OWA
The issue may be you are trying to us CAS 2010 to CAS 2007 proxy and you don’t have the correct files on the 2010 server.
To resolve
1. From the 2007 Exchange server copy the following folder to 2010:
“\Program Files\Microsoft\Exchange Server\Client Access\Owa\8.x.xxx.x“
Copy the highest numbered 8.x.xxx.x folder to the Exchange 2010 Client Access server:
“\Program Files\Microsoft\Exchange Server\V14\Client Access\Owa”
the 2010 OWA directory should look some thing like this
2. Then run IISRESET from and elevated command prompt
Troubleshooting Exchange 
excellent, i was stuck at the same issue, ur article worked
keep up the good work
Having the same error but this fix doesn’t seem to be working 😦
an you have the same events? have you done any updates to 2007?
Hi,
WHat if the upgrade was not done on the Internet facing CAS and internal CAS.
if your trying to access a 2007 mailbox across AD site boundaries via a 2010 case you will have this issue if you do not copy the files or update mentioned.
Great Solution for my current (same) issue. Well done!