Symptoms

After successfully performing the upgrade or migration, intermittently the mail handler is not creating issues even though testing the Incoming Mail seems to be functioning correctly.
Even though testing the Incoming Mail seems to be successful.

Cause

Prior to the upgrade, a testing or staging instance was created. After successfully perform the upgrade or migration, that instance was not turned off or started without the user knowing.

Resolution

Shutdown the test or staging instance, as it will also check the configured Mail Box and create issues in the staging instance instead of your Production instance.
If you are no longer need to use the testing instance, you may also remove it, delete the mail handlers, or turn off mail handling.