Disabling local mail via Plesk
1. Login to your Plesk control panel.
2. Click on Subscriptions then the name of the domain.
3. Click on the 'Mail' tab.
4. Click on 'Change Settings' then untick the 'Activate Mail Service on This Domain'.
Disabling Local Mail Using cPanel
1. Login in to WHM. Click on Account Information -> List Accounts.
2. Log into the domain's cPanel by clicking the 'cP' icon.
3. Click MX Entry from the Mail section. Now choose Remote Mail Exchanger in the Email Routing section.