Mail server blocklists

A blocklist, or DNSBL, is a list of IP addresses known for sending spam, published over DNS. Receivers query it before accepting mail. If your sending address is on one, nothing else in your setup matters much until it is off again.

bash

# the query is the address reversed, plus the zone
dig 10.113.0.203.bl.example.org A +short
# an answer means listed, NXDOMAIN means not listed

If you are listed

Find the cause before asking for removal. It is nearly always a compromised mailbox sending through your server, a web form or contact script being abused as a relay, or a mailing list someone imported without permission. Delisting without fixing the cause gets you relisted within a day, and repeated delisting requests are noticed.

Once it is fixed, use the removal form on that blocklist site. Most have a self service form and act quickly. Some lists expire entries on their own after a quiet period, so for those doing nothing is also an option.

Why a check can say "unavailable"

Some lists, Spamhaus among them, refuse lookups that arrive through a public DNS resolver and answer with a special code in 127.255.255.0/24 rather than a real verdict. That code means the query was rejected, not that the address is listed. A checker that misreads it will tell you a perfectly clean server is blacklisted. If a tool reports you as listed, confirm it on the blocklist own lookup page before doing anything.

Staying off them

Rate limit outbound mail per account so one compromised password cannot send ten thousand messages before you notice. Require authentication for submission and never relay for the world. Watch your queue size, a sudden spike is usually the first sign. And keep the reverse DNS, SPF, DKIM and DMARC in order, because a well configured sender is far less likely to be listed in the first place.