If you experience issues with general or intermittent inbound email delays from the Symantec Email Security Service, where you are using a Microsoft Exchange Server based on-premises configuration and the outbound traffic is not affected, this is most likely due to a service of the Exchange server called back pressure. Back pressure is a feature that monitors resources such as free disk space and the memory available on the system.
If one or more of the resources being monitored hits a predefined limit, Exchange will then stop the inbound mail-flow, providing temporary SMTP errors to inbound connections. This will not have any effect on the ability to send emails outbound.
Symptoms of back pressure causing email delays:
Your users cannot receive inbound emails.
The Exchange server appears to be running fine and there are no outbound mail issues.
Restarts on the server do not resolve the issue and no errors are found on the event logs.
Email Track and Trace tool results show that inbound emails are retrying with the response from your server 4.3.1 Insufficient System Resources.
Senders are receieving NDRs reporting temporary delivery failures referencing the error 4.3.1 Insufficient system resources.
Customer using an on-premises type email hosting solution, based on Microsoft Exchange Server
452 4.3.1 Insufficient system resources
Low Disk Space on the drives hosting Microsoft Exchange services
Low System Memory
NOTE: The solutions in this scenario are related to the configuration and management of the on-premises Microsoft Exchange Server solution.
Possible solutions in this scenario are:
Identify the resources which trigger the back pressure mechanism and take actions to make more of them available
Modify the back pressure thresholds, allowing the continued operation of the Exchange server within the same environment
Disable the back pressure feature
For specific details regarding your particular Exchange implementation, please reference the following official Microsoft articles: