Cannot open your default email folders Microsoft exchange is not available.Either there are network problems or the exchange server is down for maintenance.

You'll probably see Event ID 9646 is logged in the application event log of your Exchange Server 2003 computer for a client opening many MAPI sessions.

This KB relates to the error below:

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.