Environment:
Windows 2008 R2 – Exchange 2010 RTM
Later I installed SP1 & Rollup 2 for SP1
Outlook 2003 Service Pack 3 Clients
Issue:
During normal working hours users randomly was unable to access their mailboxes when they launched their Outlook client.
The users were receiving the following Outlook message: 
“Unable to open your default e-mail folders. The Microsoft Exchange Server computer is not available. Either there are network problems or the Microsoft Exchange Server computer is down for maintenance.”
It was also reported that some users had issues expanding additional mailboxes. (Delegate Mailboxes )
The following Outlook message appeared.
Or users have issues opening Shared Calendars.
Solution:
http://support.microsoft.com/kb/2299468
With Get-ThrottlingPolicy you can see the value of RCAMaxConcurrency
(Exchange 2010 RTM default value 20) (Exchange 2010 SP1 default value 214748364)
I changed RCAMaxConcurrency to 214748364 and the problem is fixt:
Get-ThrottlingPolicy | set-ThrottlingPolicy -RCAMaxConcurrency 214748364
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