A blacklist is used to defend against spam. On this list are undesirable ones:

• IP Addresses
• Email addresses
• Domains
• Web Addresses (URL)

If an email has a match on a blacklist, the email is either rejected immediately or receives points (score). The advantage of a score is that false blacklist entries can be compensated for by positive criteria. In addition, the quality of the blacklists can be assessed differently.

Blacklist entries are made after feedback from people (manually) or through automated processes (feedback from spam reviews, error messages).

For positive criteria, a whitelist is used analogously.

Further information:

• https://mxtoolbox.com/blacklists.aspx: Check whether your own server is listed on a blacklist