If you are seeing error 9548 in the Application logs, an account was probably disabled. Grant the SELF user, associated external account and full mailbox rights. Or if this is for many users, use the nomas.exe tool, but this has to be requested by Microsoft Product Support Services (PSS). See http://www.msexchange.org/articles/NoMAS-Tool.html

This will also cause mail to not be delivered to disabled user accounts. See this article for more information.

To resolve, define the "SELF" account as the associated external account in Mailbox Rights. This sets the msExchMasterAccountSid attribute.

This can be fixed on a larger scale by running an LDAP query and applying the master account sid attribute using the NoMas tool, but it is only available by request to Microsoft PSS. For more information about the NoMas-tool, refer to msexchange.org (http://www.msexchange.org/articles/NoMAS-Tool.html).