A users mailbox is being switched and then the target user account is enabled, but the mailbox is not getting the correct permissions; the user does not have permissions to his own mailbox and the permission needs to be added manually.
What is the cause?
This could have happened if the users were migrated in the past and created as disabled accounts so they did not have permissions to their own mailbox. There was an associated external account only which had permissions to this mailbox.
After the user was enabled the associated external account was removed but permissions have never been assigned correctly and Exchange Processing Wizard (DMW) or ADPW (QMM) have not been used in order to set the permissions for target accounts.
Verify if the permissions on the mailboxes are set properly.
It is a good idea to obtain the NoMAS (No msExchMasterAccountSID) tool from Microsoft premier support and use it to resynchronize all of the mailbox permissions. The tool can be used in 3 modes - check, fix and resynchronize.
The Resynchronize mode queries all mailbox-enabled user accounts and synchronizes Associated External Account and msExchMasterAccountSid.
Since the problem described above is caused by the fact that enabled users have the MSExchMasterAccountSid set the NOMAS tool can be used with the check (first) and fix (second) option on the Enabled users only.
For more information review the MS KB article 555410: Event ID 9548 is logged for Disabled User Accounts which do not have a master account SID - http://support.microsoft.com/kb/555410
which contains this advise in More Information:
If you have a large number of disabled user accounts who do not have the msExchMasterAccountSid attribute set, you have to obtain the 'NoMAS' Tool from Microsoft Product Support Services, PSS.
More information on using NoMAS is explained by Neil Hobson in his excellent article on http://www.msexchange.org/articles/NoMAS-Tool.html