After applying Cumulative Updates to Exchange released in February 2019, the Symantec Email Submission Client (SESC) no longer removes items out of the "Report Spam" or "Submit Spam" folder automatically. The items are only removed once per day on the scheduled configured for "System update" in the SESC console.
Microsoft has released a patch that removes the "Push Notification" functionality from Exchange. SESC relies on push notifications to be informed that new items are available to be processed and submitted. This prevents SESC from submitting items in near real time, and it is now capable only of retrieving items on a set schedule once per day.
For more information about the security patch, see Microsoft article Exchange Web Services Push Notifications can be used to gain unauthorized access.
Symantec email submission client is no longer being actively developed, the current situation is not scheduled to change at this time.
SESC will continue to submit items once per day, but real-time submission will no longer function going forward.