Users that have access to a number of shared mailboxes in their Outlook may notice the contents of those shared mailboxes are no longer synchronized properly. Their own personal mailbox does not seem to have this problem. On the server running the Exchange Information Store (MSExchangeIS), a variation of the following event is logged when this occurs:

Event ID 9646
Mapi session "ba765653-5439-437a-993f-806575b85fbb: /o=My Company/ou=First Administrative Group/cn=Recipients/cn=user" exceeded the maximum of 500 objects of type "objtFolder".

The reason for this error is that the Outlook client has hit the maximum number of MAPI connections allowed for the specified object   ...