Topic Last Modified: 2008-10-15

The Microsoft Exchange Analyzer Tool reads the following registry branch to determine whether the Disable Session Limit registry value is present and configured to anything other than a value of 0:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSexchangeIS\ParametersSystem

If the Exchange Analyzer finds the Disable Session Limit registry value present and configured to have a value other than 0, a warning message is displayed.

This warning means that the Exchange Server 2003 Service Pack 1 (SP1) or later versions default MAPI session limit has been disabled.

Every time that a MAPI client or application opens a session on a computer that is running Microsoft Exchange, it causes the Microsoft Exchange Information Store service (store.exe) to consume a small amount of physical memory.

To prevent MAPI client sessions from causing the Exchange Store service to allocate excessive memory, versions Exchange Server 2003 SP1 and later versions impose a restriction on the number of permitted MAPI sessions per user. By default, the maximum number of permitted MAPI sessions per user is set to a hexadecimal value of 0x20 (decimal value of 32) for Exchange 2003 SP1 or later versions.

To address this warning, set the value of the Disable Session Limit registry entry to 0, or delete the entry.

Setting the value for the Disable Session Limit registry entry to 0, or deleting it, may cause Event ID 9646 events to be logged in the application event log of the Exchange server. This is expected behavior and indicates excessive MAPI sessions by one or more client applications.

We recommend that, instead of disabling or elevating the default per user MAPI session limit, the underlying network or application issues causing the excessive sessions per user be addressed. The cause of these issues might include, but are not limited to, the following:

• General networking issues such as intermittent client network connectivity problems
  
  
• Third party add ins to Microsoft Office Outlook creating excessive sessions
  
  
• A firewall between the client and server configured to reset RPC connections ahead of the default TCP/IP KeepAliveTime value
  
  
• High latency networks such as wireless/VPN connections dropping connections
  
  
• Installation of the Windows Server 2003 Scalable Networking Pack (SNP). If you have installed the SNP, you should review and apply the following guidance:
  
  
  • Microsoft Knowledge Base article 948496 "An update to turn off default SNP features is available for Windows Server 2003-based and Small Business Server 2003-based computers" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=948496)
    
    
  • Microsoft Knowledge Base article 950224, "A Scalable Networking Pack (SNP) hotfix rollup package is available for Windows Server 2003" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=950224)
    
    
• Users opening mailboxes that contain many folders or having the Outlook 2007 Shared Folders feature enabled. See Microsoft Knowledge Base article 842022, "Event ID 9646 is logged in the application event log of your Exchange Server 2003 computer when a client opens many MAPI sessions" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=842022) for more information about this issue.
  
  

If the Disable Session Limit registry value was intentionally enabled to allow service accounts to open more sessions because of application requirements, consider using the workaround described for those accounts in the previously mentioned Microsoft Knowledge Base article 842022. The workaround should be used sparingly to avoid out of memory or virtual memory fragmentation issues.

To correct this warning by setting the value for the Disable Session Limit key to 0

1. Open a registry editor, such as Regedit.exe or Regedt32.exe

2. Navigate to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSexchangeIS\ParametersSystem

3. Right-click the Disable Session Limit registry value and then click Modify.

4. In the Value data box, type 0, and then click OK.

5. Close Registry Editor and restart the Microsoft Exchange Information Store service for the change to take effect.

To correct this warning by deleting the Disable Session Limit key

1. Open a registry editor, such as Regedit.exe or Regedt32.exe

2. Navigate to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSexchangeIS\ParametersSystem

3. Delete the Disable Session Limit registry value.

4. Close Registry Editor and restart the Microsoft Exchange Information Store service for the change to take effect.

Before you edit the registry, and for more information about how to edit the registry, see Microsoft Knowledge Base article 256986, "Windows registry information for advanced users" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=256986).

For more information about this issue, see the following Microsoft Exchange resources:

• Microsoft Knowledge Base article 842022, "Event ID 9646 is logged in the application event log of your Exchange Server 2003 computer when a client opens many MAPI sessions" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=842022)
  
  
• Maximum open message objects per logon has been changed
  
  
• Maximum open folder objects per logon has been changed
  
  
• Maximum open folder views per logon has been changed
  
  

For more information about the TCP/IP KeepAliveTime value, see Microsoft Knowledge Base article 324270, "How to harden the TCP/IP stack against denial of service attacks in Windows Server 2003" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=324270).

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