Login at the Customer Control Panel with your main account's username and password.
Once logged in, then you would make sure you’re on your “VPS” subscription at the top right hand corner.
Once you’ve made sure you’re on the correct subscription, click on the “Login via IP Address” under the Plesk section under the “Home” tab.
Once you’ve logged into Plesk, click on the Domains link on the left hand column.
Select the domain you want to manage.
Click on Email addresses icon.
There you can delete or add email accounts as needed.