Mail server refusing emails with error "421 4.4.2 Message submission rate for this client"

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.

1.  Open the MTA log, search for error, if you see an error like - 421 4.4.2 Message submission rate for this client has exceeded the configured limits then follow the next step.
2. Go to exchange management shell type Get-receiveconnector | ft Name, messageratelimit without quotes and hit enter. This will show you the max hop count value for each receive connector.
   
3. Type following command in management shell without quotes - Get-receiveconnector -Identity | Set-receiveconnector -messageratelimit unlimited.
   
4. Restart services on email security from system/ host configuration. Once the email security comes back online it will start delivering those queued emails automatically.

See also:

Resource Monitoring and 452 4.3.1 Insufficient system resources in Exchange 2007 and SES