Greylisting is a service provided by cPanel/WHM to stop receiving spam and unwanted incoming mails. On enabling this option, the mail server will temporarily reject all mails from any sender (other than from the trusted mail servers) in the first attempt.
Legitimate sender mail server would retry sending the mail after a short delay which will be checked by the receiving mail server in which Greylisting is enabled and only legit mails are passed through after analyzing triplets.A triplet consists of the IP address, the sender’s address, and the recipient’s address. By checking all these three parts, Greylisting tool filters spam emails and allows only legitimate emails to pass through.
How to enable Grelisting.
Just click on the ‘On/Off’ toggle and click ‘Save’