This KB relates to the error below: http://support.microsoft.com/kb/842022
However, i found that moving the users mailbox to another store resolved this issue without making registry changes.
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
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.