Exchange 2003 issue after changing external DNS mx record

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http://serverfault.com – We currently use an external spam filter service that forwards our incoming mail to our internal exchange server. We have decided to stop this service and use Sophos pure messgage on our exchange server instead. After requesting the external dns mx record to point to our exchange servers public ip (via NAT) instead of the external filter, internal outlook 2007 clients were unable to connect to their mailboxes and owa would mot connect to the mailboxes either. I quickly reversed the external change, rebooted he exchange server and all was ok again. Why were the clients \ owa unable to connec (HowTos)