Mailboxes have not been synchronizing. When viewing the MSA (Mail Source Agent Log) the following error is displayed.
Error when attempting to switch a mailbox. CMBRedir::MakeRedirector Error -2147023080 LDAP error 0xb. Administration Limit Exceeded (00002024: SvcErr: DSID-02050847, problem 5008 (), data -1026 Could not find error in Quest Knowledgebase.
This is caused by the AD object being processed having too many addresses in the proxyAddress attribute.
Open up the ADSIEdit and connect to the source Active Directory. Find the user account that the MSA log is referring to and clear out unused addresses in the proxyaddress attribute.