How to check if your IP is on an email blacklist

Blacklists (or blocklists) are lists of IPs considered spam sources. If your IP ends up on one, your emails are rejected or marked as spam.

1. Check the IP

There are free online services that check dozens of known blacklists at once. You enter the server IP and see which lists it appears on.

2. Check from the terminal

You can query a blacklist manually (generic example):

dig +short 10.113.0.203.zen.spamhaus.org

A reply means the IP (reversed here) is listed. No reply means it is not.

3. What to do if you are listed

• Find the cause: a compromised account, a script sending spam, or an IP with a bad history.
• Fix the problem (change passwords, stop the script).
• Request delisting on the blacklist's website.

4. Prevent

• Configure SPF, DKIM and DMARC correctly.
• Set reverse DNS.
• Filter outgoing spam, see spam filtering.