This interface in your cPanel allows you to configure Greylisting, a service that protects your server against unwanted email or spam. When enabled, the mail server will temporarily reject any email from a sender that the server does not recognize. If the email is legitimate, the originating server tries to send it again after a delay. After sufficient time passes, the server accepts the email.
Greylisting identifies incoming email by triplets. A triplet is a collection of three pieces of data: the IP address, the sender's address, and the recipient's address. By deferring unknown triplets, Greylisting filters spam and allows legitimate email a second chance to pass through.
This interface only displays an On/Off toggle. Click Off to move the toggle to On and enable Greylisting.