When SpamExperts tries to deliver and email but gets rejected for whatever reason it records the fact. In future, SpamExperts will automatically reject messages sent to this mailbox without checking your cPanel account for several hours. This is to minimise network traffic. This list is not maintained forever, and the cache does expire wiithin a few hours.
It's important in these circumstances to clear the callout cache so any changes you've made to correct Destinations are taken into account immediately.
Accessing SpamExperts
cPanel > Email > SpamExperts
On the next screen, you will see a list of domains within this cPanel account - click 'Login' to enter the SpamExperts configuration for that domain
Clearing the Callout Cache
Click the Clear Callout cache - incoming icon.
Then click the Clear button.
You'll see a success message - the cache is cleared. Please re-test inbound email to confirm it is now being successfully received.