Black List
Blacklisting is a means of controlling access to a service or system, or a means of restricting those on the "blacklist" from contacting those on a particular email system. Blacklists can relate to individuals, or in the case of firewalls and access control systems, they can relate to websites or entire domains.
White List
A white list is the opposite of a blacklist. It is a means of allowing a select list of people (in this case, email from people) through an access control system. Messages from those on a white list would pass the blocking mechanism, while those not on the white list would be sent to trash or bounced.
Grey Listing
Grey listing (or gray listing) is a system by which an email is received and bounced back to the server. If the email is then resent by the sender, the system will then allow the email to be sent through to the recipient. This is based on the assumption that a spammer will not resend email, and so only legitimate email will be retried by the sender.
This system of temporary rejection is a commonly used antispam mechanism.
