Graylisting is a technique used by some mailbox providers and email receivers to thwart spammers.
A receiving mail server that uses graylisting temporarily rejects any of your email if the server does not recognize you as a sender. The receiver presumes that if you are a legitimate sender, the originating server will try again later to send the email, at which time the receiver will accept it. Graylisting presumes that if you are a spammer, you will not resend your message later.
Email Glossary
