The Gray List or Greylisting is a very effective AntiSpam technique that consists of temporarily blocking certain unknown senders. This may cause you to receive some emails with a certain delay, so if the delay is not acceptable, all you have to do is deactivate the greylist for your domain(s).
To access manage this feature, you must first access your Cpanel.
Once in your CPanel, you must access the Greylisting area, in the email section.
Within this section you will find a list of the domains available in the plan. You can activate or deactivate greylisting for each of them.