To suspend emails for whole domain in Plesk follow the following steps:-
1) Login to Plesk Page.
2) Click the "Domains" shortcut in the navigation panel.
3) Click the required domain name in the list.
4) Go to Mail Accounts.
5) Click Mail Settings
6) Uncheck the box "Activate mail service on domain"
7) Click on "OK"
The emails for the whole domain has been disabled now.