Recently I had an issue with all distribution lists on Exchange 2013 Server. All E-Mails send to DL are Stuck in Mailqueue with error : : 451 4.4.0 Primary target IP address respondet with “421 4.3.2 Service not available”
Used get-ServerComponentState -Identity servername . and found that the “Hubtransport component”is in draining state.
The issue was resolved with:
Set-servercomponentstate –component Hubtransport –state Active –requestor maintenance
For more infromation: