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:
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:
For specific details regarding your particular Exchange implementation, please reference the following official Microsoft articles: