With Plesk, you can configure your server to use external mail servers, such as Google® and Yahoo!®, to handle email services.
Once complete, your server sends email from your Website, and other applications installed on it, but no longer receives email for the domain name you have disabled email on.
When you disable email for a subscription in Plesk, it affects all of the domains in that subscription.
If you want to allow email on one domain and disable it for another, they need to be in different subscriptions.