KB ID 0000803
421 4.3.2 Service not available
Connection to host lost.
Normally I’d expect to see an error like this if the Exchange ‘Receive Connector’ was misconfigured, (if you’re not using SBS 2011 create a new default receive connector and make sure there are no firewalls in the way).
If you are using SBS 2011 you need to run the ‘Connect to the Internet’ Wizard (seriously!)
After this check inbound mail flow again.
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