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

On a server that is running Microsoft Exchange Server 2003, an event that resembles the following event is logged in the Application log:

Event Type: Error
Event Source: MSExchangeIS
Event Category: General
Event ID: 9646
Description:
Closing Mapi session "/o=Organization/ou=Administrative Group/cn=Recipients/cn=Recipient" because it exceeded the maximum of 32 objects of type "session".

When this issue occurs, you may also receive the following error message in Microsoft Office Outlook 2003:

Unable to open your default e-mail folders. The Microsoft Exchange Server computer is not available. Either there are network problems or the Microsoft Exchange Server is down for maintenance.

1. Click Start, click Run, type services.msc in the Open box, and then click OK.
2. Click the MSExchange Information Store service, and then click Restart Service.