Protecting email users from spam is the purpose of greylisting. To temporarily ignore messages from unrecognised senders, you can turn on greylisting on your mail server. The original sending server will, if the email is genuine, try sending it again after a time. The server will recognise the email as legitimate when a certain length of time has passed.
- Login to your WHM panel.
- Select Email on menu bar and then click on Greylisting.
- Click the On/Off button to enable Greylisting.
- Do all the configration as needed and then click on SAVE button
Read More:- How to Restart PHP-FPM WHM?