Greylisting is a way to reduce spam by temporarily rejecting emails from senders your server hasn’t seen before. If the email is from a legitimate source, the sender's server will try again after a short delay, and the message will then be accepted. Spam servers usually don’t resend, so their messages get blocked. The system recognizes senders using a combination of their IP address, email address, and your email address to decide whether to allow or delay the message.
This command checks each cPanel user on the server and disables Greylisting for all the domains they own.
for i in $(\ls /var/cpanel/users) ; do uapi --user=$i cPGreyList disable_all_domains ; done