Event ID 9646 is logged in the application event log of your Exchange Server 2003 computer when a client opens many MAPI sessions

To change the value of the maximum permitted MAPI sessions per user from the default, you can configure the Maximum Allowed Sessions Per User registry entry. To do this, follow these steps.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

322756  (http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and then click the following registry subkey:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem

3. If the Maximum Allowed Sessions Per User entry does not exist, do the following:
   1. On the Edit menu, point to New, and then click DWORD Value.
   2. Type Maximum Allowed Sessions Per User as the entry name, and then press ENTER.
4. Right-click the Maximum Allowed Sessions Per User entry, and then click Modify.
5. Click Decimal, type the value that you want to set in the Value data box, and then click OK.
6. Exit Registry Editor.
7. Click Start, click Run, type services.msc in the Open box, and then click OK.
8. Click the MSExchange Information Store service, and then click Restart Service.