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