Greylist

With grey listing what is meant is that something (a policy) is recognized to be neither White nor Black. Watching the behaviour of a greylisted policy can decide whether to later Whitelist or Blacklist it. In OpenBSD's spamd for example a mail delivery is rejected until a certain amount of time has passed and the remote end has tried to deliver the mail again. This will whitelist the sender after some time because it's assumed that if the person is a spammer they'll not try to deliver the mail again to the same IP (ie. a spamming virus).