Email Grey Listing
Grey Listing which is an effective spam filter technique is a feature that temporarily rejects an e-mail from an unknown sender. The sender is held within the grey list record, typically for a time limit around 24 hours. Legitimate e-mail’s will automatically retry sending the message, and the spam filter which grey listed the e-mail will now accept the mail as legitimate.
Grey listing has been put into affect in spam filter software as many spammers use spambots which attempt to send spam to many recipients, but if the spam email is not accepted, spam sources do not retry sending the same message again. They continue to send bulks of spam to other sources. It is too time consuming to wait around and try resending a message for these spambots. So grey filtering does contribute well to stopping spam.
Wikipedia's guide to Grey Listing