Current Status: Issue with mitigation

Exchange 2010 SP1 together with the resolutions mentioned later in this section, allows you to open as many as 16 shared calendars or additional mailboxes simultaneously independent on whether the mailboxes are located on Exchange 2003, 2007, or 2010. If you have more than 16 calendars or additional mailboxes opened, you may randomly see error message similar to the one shown in Figure 4.

Figure 4: Error message when opening more than 16 calendars

With Exchange Server 2010 RTM deployed into an Exchange 2003 or Exchange 2007 organization, it was a common issue that when an Exchange 2003 or Exchange 2007 user tried to open more than two additional Exchange 2010 mailboxes or shared calendars using Outlook 2003, she would receive one of the following error messages:

• The set of folders could not be opened
• The information store could not be opened
• Unable to display the folder. The information store could not be opened

When an Exchange 2007 user tried to send an e-mail using Outlook 2003, she would sometimes also receive the following error message:

• Task ‘Microsoft Exchange Server – Sending’ reported error (0x800C8100): ‘Unknown Error 0x800c8100’

These issues were resolved with Update Rollup 2 for Exchange 2007 Service Pack 2 and a hotfix that were released for Exchange 2003 SP2. More information about the issues and how they are resolved can be found in the following KB articles:

• Office Outlook 2003 does not connect to two or more additional mailboxes in a mixed Exchange Server 2007 and Exchange Server 2010 environment
• An error occurs when an Exchange server 2003 user tries to open more than one delegate mailbox of Exchange Server 2010 in Outlook 2003

Although the above mentioned issues were resolved, some customers, partners, and individuals in the Exchange communities reported they still experienced issues when trying to open approximately multiple shared calendars and/or additional mailboxes using Outlook 2003.

For most organizations, the issue can be remediated by installing Exchange 2010 SP1 as this service pack includes a fix that makes it possible for an Exchange 2003, 2007, or 2010 user to open as many as approximately 16 shared calendars or additional mailboxes using Outlook 2003.

Figure 5: By default approximately 16 Calendars can be opened using Outlook 2003

If you have users that needs to open more than 16 shared calendars or additional mailboxes using Outlook 2003, you can adjust the RPC related throttling policy settings using the Set-ThrottlingPolicy cmdlet. Specifically, you need to increase the value for “RCAMaxConcurrency” which by default is set to “20”. The RCAMaxConcurrency parameter indicates how many concurrent connections an RPC Client Access user can have against a server running Exchange 2010 at one time.

Figure 6: Default setting for the RCAMaxConcurrency throttling policy value

For instance, to increase the value of the “RCAMaxConcurrency” setting in the default throttling policy from 20 to 2147483647, open the Exchange Management Shell and run the following command to first create a variable for the policy:

$a = Get-ThrottlingPolicy | where-object {$_.IsDefault -eq $true}

Then pipe the variable to the Set-ThrottlingPolicy commandlet:

$a | Set-ThrottlingPolicy -RCAMaxConcurrency 2147483647

Figure 7: Increasing the value for the RCAMaxConcurrency throttling policy setting

In order to apply the changes, restart the “Microsoft Exchange Throttling” service on each CAS server in the organization.

You can read more about Exchange 2010 SP1 throttling policies in the Exchange 2010 documentation on Microsoft TechNet.

If you still have issues opening shared calendars or additional mailboxes, you may want to increase the value of the RCAMaxConcurrency throttling policy setting to 100 or even higher. Read more in Error message when Outlook 2003 clients try to open multiple shared calendars in Exchange Server 2010: "The connection to the Microsoft Exchange server in unavailable. Outlook must be online or connected to complete this action".

If you see event 4696 with a description similar to the following logged in the application log on the Mailbox servers in the organization:

"Mapi session "00cc8dde-64d7-4353-8050-00fc2057aae3: /O=xxxx/OU=xxxx/cn=Recipients/cn=ward" exceeded the maximum of 32 objects of type "session"."

You need to increase the maximum allowed sessions per user and/or maximum allowed service sessions per user limit from "32" to "64" or even higher. See more information at: Exchange 2010 SP1 Store Limits.

but when I tried to add the “szMaxAllowedSessionsPerUser” and/or “szMaxAllowedServiceSessionsPerUser”, I still saw 9646 in the app log.

Guess why? yes the registry keys are actually listed with wrong names in that article. Instead of:

• szMaxAllowedSessionsPerUser
• szMaxAllowedServiceSessionsPerUser

You need to use:

• Maximum Allowed Sessions Per User
• Maximum Allowed Service Sessions Per User

And then everything worked as expected…

Hopefully the TechNet page is updated soon.

Special Thanks to Henrik Walther