Greylisting is a security measure that protects your cPanel email users against spam by temporarily rejecting any email messages from an unknown sender.
In case you are not receiving emails from a specific domain, you can disable greylisting from cPanel hosting. Just go to your cPanel Email section and choose Configure Greylisting:
Select the domain for which you want to disable greylisting and change the status to Off:
That's it! You will start receiving all email messages from any domain to your inbox.