This Error event indicates that a MAPI session tried to open more than the maximum number of objects that are allowed for the object type specified in the event description. The MAPI session that caused this event is specified in the event description.

To resolve this problem, follow these steps:

1. Investigate any third-party applications or add-ins that are running on the computer of the user who is specified in the event description of event MSExchangeIS 9646. If the third-party application or add-in typically opens too many objects on the server or leaves too many objects open on the server, other clients may be unable to log on to the server. For more information, see Microsoft Knowledge Base article 830829, Your Exchange Server 2003 computer may stop responding after a MAPI client opens more than the default value of certain server objects.
   
   
2. Investigate the functionality of any batch file or scripts that you may use to send messages. If the batch file or script does not correctly close MAPI sessions, other clients may not be able to log on to the server. For more information, see Microsoft Knowledge Base article 830836, Event ID 9646 is logged when you try to send many messages in Exchange Server 2003.
   
   
3. Monitor the MAPI session that is specified in the event description to exclude any unusual activity associated with this MAPI session.
   
   
4. Increase the default maximum limit for the object type that is specified in the event description by modifying the registry. To do this, follow these steps:
   
   Important   Use caution before you adjust the maximum number of objects that are allowed per MAPI session. When you increase the maximum number of an object type, you also increase the memory that may be consumed by MAPI requests. Configuring this value incorrectly can cause out-of-memory warnings or virtual memory fragmentation warnings. Increasing this value can also prevent any e-mail clients from logging on to the server.
   
   Caution   Modifying the registry incorrectly can cause serious problems that may require that you reinstall the operating system. You might be unable to resolve problems that are caused by modifying the registry incorrectly. Before you modify the registry, back up any valuable data.
   
   
   1. Click Start, click Run, type regedit, and then click OK.
      
      
   2. Expand the following registry subkey:\\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
      
      
   3. Right-click ParametersSystem, point to New, and then click Key.
      
      
   4. Type MaxObjsPerMapiSession, and then press ENTER to name the new sub key.
      
      
   5. Right-click MaxObjsPerMapiSession, click New, and then click DWORD Value.
      
      
   6. Enter the object type as specified in the event description, and then press ENTER.
      
      
   7. Right-click the object type that you entered, and then click Modify.
      
      
   8. In the Value data box, type the number of objects to which you want to limit this entry, and then click OK.
      
      

The following list provides default values for each object type in Exchange Server 2007.

If you are not already doing so, consider running the Exchange tools created to help you analyze and troubleshoot your Exchange environment. These tools can help make sure that your configuration aligns with Microsoft best practices. They can also help you identify and resolve performance issues, improve mail flow, and better manage disaster recovery scenarios. To run these tools, go to the Toolbox node of the Exchange Management Console. To learn more about these tools, see Managing Tools in the Toolbox.