Final answer:
A blacklist is a compilation of servers that are blocked because they have been known to send spam. It is used to protect users from receiving spam or potentially malicious emails.
Step-by-step explanation:
A blacklist is a compilation of servers that are blocked because they have been known to send spam. When a server is blacklisted, it means that its IP address is added to a list of known spam sources, and emails originating from that server are often automatically marked as spam or blocked by email filters.
Blacklists are used by email service providers and network administrators to protect users from receiving spam or potentially malicious emails. These lists are continuously updated to include new sources of spam and remove servers that have resolved their spam issues.
An example of a well-known blacklist is the Spamhaus Block List (SBL), which is used by many email service providers to filter out spam.