01) Access your cPanel account
02) After login click on Email >> Configure Greylisting
03) In the domain to disable click on Off and to activate click on Enabled
Graylist or gray list is an existing technology for mail (SMTP) servers. In order to avoid receiving SPAM, this technology can be used in conjunction with other blocking technologies. It basically consists of temporarily blocking incoming mail for a period of time. After this, a new attempt to deliver the email is accepted in the short term. If the email is delivered again within this opening period, the server's internet address and email domain is added to a mailing list, and it is only expired after a long period of non-use. If no further attempt is made within this short time, the email is blocked again in the future attempt.
It sounds like an unfounded technology, but it relies on the fact that servers delivering millions of bulk messages do not have the ability to be resubmitted to all emails that have failed temporarily.
Learn more about Greylisting, go to: