Mail sync fails for certain mailboxes. The MSA log shows the following:
8/16/2012 11:36:09 AM CMBMConn::SetMailboxInfoE2K TraceMsg 6270 Moving mailbox '/o=Org/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=First.Last' in progress. Check move request wasn't executed because time hasn't approached.
8/16/2012 11:36:13 AM PowerShellCmdExec::CheckMoveRequestCompleted Error 6219 Move mailbox request 'NM60628' completed with errors. Error code: '-2146233088'. Status: 'Failed'. Message: Error: The move restarted because the signature of the source mailbox changed. --> MapiExceptionCannotRegisterNewReplidGuidMapping: IExchangeFastTransferEx.AddMdbEvents failed (hr=0x80004005, ec=2620)
The move mailbox request in question has been created a long time ago (in this particular case, it was two months old)
Open the Exchange Management Console
Delete the problematic mailbox move request
The MTA should generate a new mailbox move request and process the mailbox successfully