12/20/2019 1,086 People found this article helpful 467,515 Views
If your emails are not being delivered and you notice that emails are queued on email security MTA queue under Reports & Monitoring / Monitoring /MTA Status. You should check PMTA-MTA:Log. You can download this log from system/advanced page.
Mail server may reject emails with error message - "421 4.4.2 Message submission rate for this client has exceeded the configured limits" if the hop count crosses the limit specified on receive connector properties.
NOTE: In case of email security the sender will not receive an NDR because message will stay in MTA queue and it will retry to deliver the message.
You can follow this Microsoft kb article to resolve this problem - Setting the Message Hop Count.
See also:
Resource Monitoring and 452 4.3.1 Insufficient system resources in Exchange 2007 and SES