Currently we have emails going through the SMG then to Exchange Server 2013. The problem I'm having is every so often(2-3hrs) messages get stuck in the Delivery Queue with error 421 4.4.0 "no Mail server for this domain could be reached at this time" I restart the Exchange Transport service on the Exchange Server, then flush all messages in Queue and email flows again. Does anyone have any idea what could be causing this? also now I have a bunch messages sitting in the Queue with error 451 4.4.1 "Connection was not attempted: domain is marked down (too many Connection Failures)" says their from "MAILER-DAEMON" and to External email addresses.
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