An email blacklist, now usually called a blocklist, is a list of IP addresses or domains known to send spam. Mail servers check them, usually through DNS, before accepting a message. A listing can mean rejected email or more spam-folder placement.
The main kinds
- IP blocklists list sending servers, for example Spamhaus ZEN, Barracuda and UCEPROTECT.
- Domain blocklists list domains seen in spam, either as the sender or in links.
Some lists matter a lot (most providers use Spamhaus); others are used by few receivers and matter less.
How senders get listed
Hitting spam traps, many spam complaints, sudden high volume from a new domain, or sharing an IP with someone who does.
Getting removed
Find the cause first: stop sending from the affected domain or mailbox, clean the list, then use the list's own removal page. Most remove a listing once the problem stops; some do it automatically after a while. Check a domain or IP with the free blacklist checker.