In order to disable your email service in Plesk, take the following steps:
1. Log into myTophost Panel and then select Plesk Login in order to connect to the Plesk of the plan that you are interested in.
2. In the tab Mail, select Mail Settings.
3. Select the domain name that you want and click on Activate/Deactivate Services.
4. In the pop-up window, in the section Mail service, select Disable from the drop-down menu and click on OK.
Once you have disabled the Email service, you will see the following: