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.