The steps to disable your mail service for a domain are as follows:
1. Login to your Plesk panel and select the required domain you want deactivated.
2. Select the required domain.
3. Select ‘Mail’ tab and go to 'Change Settings'.
4. Click on ‘Activate mail service on domain’ to disable the option and select ‘OK’.
Note: If the domain is using an external mail server and if your contact forms are sending mail to @domain.com, than you will have to disable this service to receive mail in the external mail server.
