Deliverability
The secret lists that decide whether your email exists
Your message can be perfect and still never reach an inbox, because of a list you have never heard of.
Most mail servers ask a simple question before they accept a message: has this sender been bad before? The answer comes from a DNS-based blocklist, a list of IP addresses and domains that someone has marked as spam sources.
How a blocklist works
- The server takes the sending IP address, reverses it and looks it up in a DNS zone run by the list maintainer.
- An answer means listed. No answer means clean. The whole check takes milliseconds.
- Some lists block on sight. Others only lower the score, so the message lands in spam.
Who maintains them
- Spamhaus, a non-profit, runs several lists: spam sources, exploited machines and a policy list of dynamic home IP ranges.
- Barracuda runs a reputation list as part of its security business.
- SORBS and SpamCop run other public lists, with their own rules for removal.
- Large providers also keep private reputation scores. Their lists are not public, and they are the ones that matter most for Gmail or Outlook.
Why providers depend on them
- Nobody can review every message. A list lets a server reject a connection before the email is even read.
- Small and mid-size providers often use public lists directly. Large ones combine them with their own data.
- The result: one listing can cut deliverability sharply for a sender, and it can spread from one server to many in a day.
What to do if you are listed
- Find out which list it is, and read its removal rules first.
- Fix the cause (a compromised box, an open relay, a bad list you bought). Listings for dynamic addresses are removed only when the address is no longer dynamic.
- Never buy a list and never mail people who didn't ask.