We have some issue with our AD lately and our email queue with Unrachable Domain status. After stable, we need to resubmit back the message. Suspend and resume not work and need to be done using below command :
Retry-Queue -Identity "<ServerName>\Unreachable" -Resubmit $True
Then all message will be resubmit again. 🙂
Source : http://technet.microsoft.com/en-us/library/aa995987(v=exchg.80).aspx