Steps for enabling grey listing for the server via plesk
1. Login to the plesk panel
2. Select 'Tools & Settings' under 'Server Management'
3. Select 'Spam Filter Settings' under the option 'Mail'
4. Enable the option 'Switch on server-wide greylisting spam protection'
5. Click 'OK' button
Note: Enabling greylisting can significantly reduce spam at the expense of causing delays in legitimate email delivery.