How a DNS blocklist works
A DNS blocklist is a zone that answers for the addresses it lists. To check 203.0.113.5 against a list, a mail server reverses the address, appends the zone and asks DNS for 5.113.0.203 plus the zone name. An answer in 127.0.0.0/8 usually means listed, and the exact address says why; a few codes mean the list refused to answer, see below. No such name means not listed. Domain lists work the same way with a domain name in place of the reversed address, and they are mostly used against the domains in links and in the From line.
Each list has its own criteria and its own weight. Some are used by large providers to reject mail outright, others only add a point or two to a spam score.
When a list does not answer
Lookups fail. A list is slow or down, or it refuses queries from the network asking. Spamhaus, for example, answers queries from large public resolvers with a refusal code instead of a result. A tool that reads any of that as not listed gives a clean result exactly when it knows nothing. Here a list that did not answer is shown apart and never counted as clean.
If you are listed
Start from the code next to the listing. It names the reason the list gives: a spam source, a phishing domain, an abused legitimate domain, a poor reputation level. That points at the cause, and the cause has to go before you ask for removal. On a shared server it is often another customer. On your own it may be a compromised account or a form that sends mail to any address typed into it. Every list publishes its own removal procedure on its website, and some remove an address automatically after a quiet period.
Questions
How do I check if my IP is blacklisted?
Put the IP address your mail leaves from into the form above. You will find it in the Received headers of a message you sent, or in your mail provider's settings. All lists are queried at once, and the answer of each one is shown, the ones that did not reply included.
Should I check my IP or my domain?
Check both, and separately. The IP is the server your mail leaves from, which you can find in the Received headers or in your provider's settings. The domain matters for the domain lists, which look at the From line and at every link in the message.
My provider sends from shared IPs. Is a listing my problem?
It affects your mail, so yes, but the fix is the provider's. Send them the result and ask them to move you to a clean address or get the listed one removed.
Is one listing a reason to panic?
Look at which list it is first. A listing on one of the major lists blocks mail at many providers; a listing on a small list that few receivers consult may change nothing you can measure.
What do Spamhaus ZEN, PBL and DBL listings mean?
ZEN is the combined Spamhaus list for IP addresses: SBL with its CSS part for spam sources, XBL for infected and hijacked machines, and PBL for address ranges that should not send mail directly. A PBL listing is not an accusation. It usually means the address is a home or dynamic one, and the fix is to send through your provider's mail server. DBL lists domains seen in spam or phishing links and in From lines.
How do I get my IP off a blacklist?
Fix the cause first, or the address comes back. Then look the address up at check.spamhaus.org: the result names the list that has it and how removal works for that list. A static address in PBL can be removed there by whoever uses it, if the network's policy allows it. UCEPROTECT Level 1 drops an address on its own once it has sent no spam for seven days. Check again here after the removal goes through.
Why is my domain blacklisted?
Domain lists such as Spamhaus DBL, SURBL and URIBL add a domain when it shows up in spam as a link or a landing page, and Spamhaus DBL also looks at the sender's domain. The usual causes are a hacked website used to host phishing and a link to your site in someone else's spam. A campaign of your own that drew complaints can put the sending domain on DBL too.