The credit for the following article goes to Carter Tan, Microsoft Premier Field Engineer, and BlackBerry (the proudly Canadian company) as well. Thank for putting all this together Carter !
From Exchange 2010 SP1 and later, the MaxSessionsPerUser is set via Client Access Throttling Policy:
<snip>
Increase the maximum number of connections to the Address Book service
in Microsoft Exchange 2010
By default, Microsoft® Exchange 2010 limits the maximum number of connections from the BlackBerry® Enterprise Server to the Address Book service to 50. To permit the BlackBerry Enterprise Server to run, you must increase the number of permitted connections to a large value (for example, 100,000).
Before you begin: You must complete this task if the messaging server that the BlackBerry Enterprise Server uses is Microsoft Exchange 2010. If the messaging server is Microsoft Exchange 2010 SP1, you can change the MaxSessionsPerUser settings using client throttling policies.
1. On the computer that hosts the Microsoft Exchange CAS server, in <drive>:\Program Files\Microsoft\Exchange Server\V14\Bin, in a text editor, open the microsoft.exchange.addressbook.service.exe.config file.
2. Change the value of the MaxSessionsPerUser key to 100000.
3. Save and close the file.
4. Restart the Address Book service.
</snip>
The Client Access Throttling Policy recommended by RIM is as follows:
<snip>
Turn off client throttling in Microsoft Exchange 2010
By default, Microsoft® Exchange 2010 uses client throttling policies to track the bandwidth that each Microsoft Exchange user consumes and enforce bandwidth limits as necessary. The policies affect the performance of the BlackBerry® Enterprise Server, so you should turn off client throttling for the Windows® account that has a Microsoft Exchange mailbox.
1. On a computer that hosts the Microsoft Exchange Management Shell, open the Microsoft Exchange Management Shell.
2. Type New-ThrottlingPolicy BESPolicy.
3. Type the following command:
Set-ThrottlingPolicy BESPolicy -RCAMaxConcurrency $null -RCAPercentTimeInAD $null -
RCAPercentTimeInCAS $null -RCAPercentTimeInMailboxRPC $null -EWSMaxConcurrency $null -
EWSPercentTimeInAD $null -EWSPercentTimeInCAS $null -EWSPercentTimeInMailboxRPC $null -
EWSMaxSubscriptions $null -EWSFastSearchTimeoutInSeconds $null -EWSFindCountLimit $null
4. Type Set-Mailbox "BESAdmin" -ThrottlingPolicy BESPolicy.
</snip>
The MaxSessionsPerUser value in the Address Book Service is limited by the RCAMaxConcurrency value, which also controls the number of MAPI connections allowed by the service account.
<snip>
The RCAMaxConcurrency parameter specifies how many concurrent connections an RPC Client Access user can have against a server running Exchange 2010 at one time. A connection is held from the moment a request is received until the connection is closed or the connection is otherwise disconnected (for example, if the user goes offline).
If users attempt to make more concurrent requests than their policy allows, the new connection attempt fails. However, the existing connections remain valid. The RCAMaxConcurrency parameter has a valid range from 0 through 2147483647 inclusive. The default value is 20. To indicate that the number of concurrent connections should be unthrottled (no limit), this value should be set to $null.
</snip>