What does greylisting mean?
Greylisting is a method used by mail servers to protect a recipient from spam. To greylist a sender is similar to placing a sender on a blocklist – but the key difference is that a greylist is only a temporary measure.
Senders on a greylist will not be able to contact a recipient for a set amount of time. After that set amount of time has elapsed, the sending server will resend the message which will be received in an inbox.