When accessing the WHM module Greylisting's "Trusted Hosts" tab the following error is generated via the UI:
Failed to check if server netblock is added to the cPGreyList trusted hosts: WHOIS lookup failed
This issue is related to your server's ability to query WHOIS which utilizes the outbound TCP port 43.
Please ensure that the outbound TCP port 43 has been enabled in the server's firewall and networking. We have the following documentation covering how to configure your firewall for cPanel services: