How to disable mail service for one domain?
A domain uses an external mail service, and you need to turn off the Parallels Plesk Panel (PP) mail service for this domain to prevent local delivery. Or, you need to enable mail service for one domain of a subscription. For that followi instruction below:
Go to Home -> Subscriptions-> subscritption_name -> Mail -> Mail Settings .
Select a required domains and click on Activate/Deactivate Services button.