Email Blacklist
An email blacklist, or blocklist, is a database of IP addresses, domains or URLs flagged for spam. Listed senders can see mail rejected or filtered.
An email blacklist, now more often called a blocklist, is a database of IP addresses, domains or URLs that an operator has flagged for spam or abuse. Receiving mail servers can query these lists and reject, spam-folder or scrutinize mail from listed senders.
Types of blocklists
Some lists track sending IPs, some track domains, and some track URLs that appear in message bodies. Each has its own listing criteria and removal process. That is why one sender can be listed on one and clear on another, and why mail can fail at one provider while landing at another.
Check the operator’s published listing criteria before deciding what to change. A listing on one service does not establish why another provider filtered a message.
Common mistakes
Learning about a listing from bounce messages instead of monitoring for it. Check the rejection text from the receiving server, which may name the list, then follow that operator’s removal process. The other is requesting delisting without fixing what caused it, which can lead to being listed again.
The email deliverability guide covers the sending practices that keep a domain off these lists, and why emails go to spam covers filtering that happens without a listing.