Senders who are not blacklisted (excluded) or whitelisted (accepted) can be placed on a greylist. Some anti-spam software can send greylisted addresses an automated response, challenging the sender to confirm their legitimacy.

A list of email addresses that have attempted to send messages, and have been sent a reply asking them for information to confirm that their email is not spam.

The midpoint between a whitelist and a blacklist. Senders not on the blacklist or the whitelist may be placed on a greylist.

A mail transfer agent which uses greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later, at which time the destination will accept it. If the mail originates from a spammer, the spammer will probably not resend it. Source: Wikipedia, shortened Page top

Greylisting is a simple method of defending electronic mail users against e-mail spam. In short, a mail transfer agent which uses greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later, at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried