Spamhaus SBL and XBL, Barracuda and SpamCop are the listings that change whether your mail arrives. A scanner reporting three hits out of two hundred lists has quite possibly reported nothing that matters, and panic at that point wastes a day.
Trap hits are the most common real cause, and they point at collection practice rather than at sending. A purchased list, a scraped file, or an old segment nobody sunsetted will each produce them.
Complaints are the second. Crossing 0.3% at Google or Yahoo triggers filtering directly, and a sustained rate feeds listings elsewhere. Check your complaint rate by segment before assuming the whole list is the problem.
Compromise deserves ruling out early. A cracked mailbox password or a vulnerable form that sends mail can push spam through your infrastructure for days before anyone notices, and the listing is the first symptom many senders see.
Fix the cause before requesting removal. Delisting with the problem still running gets you relisted faster and usually with a longer cooldown, since the operator has now seen you ask without acting.
Checking a listing by hand in one command
Scanners are convenient, but you can ask a blocklist directly. Say your sending IP is 203.0.113.45. Reverse the four numbers, add the list's zone, and look it up: dig +short 45.113.0.203.zen.spamhaus.org.
No answer means you aren't listed. An answer of 127.0.0.2 means the SBL, the list for spam sources. 127.0.0.4 means the XBL, which covers hijacked and infected machines. Replies of 127.0.0.10 or 127.0.0.11 are the policy list for addresses that shouldn't send mail directly, and home connections land there by design.
One catch trips people up. Spamhaus refuses queries that arrive through big public DNS resolvers, and you may get an error code such as 127.255.255.254 where you expected a real reply. Run the lookup from your own server or your office network before you believe a result.
IP listing or domain listing: the fix is different
Find out first whether the listing names an IP address or your domain. An IP listing on a shared sending platform may have nothing to do with you. Another sender on the same address caused it, and the fix is a message to your platform's support team. They can move you to a different pool or take it up with the list.
A domain listing is personal. It follows you to any platform you move to, because the list has flagged the name that appears in your links and your From line. Changing provider won't help. You have to find the cause.
Say you see a listing on Monday and can't explain it. Check your sent volume for the past week against your normal level. A spike you didn't plan, often at night, points to a hacked mailbox or a form being abused. Change passwords and close that hole before you touch a removal form, or you'll be listed again within days.
Related questions
Which blacklists actually affect delivery?
Spamhaus SBL and XBL above all, then Barracuda and SpamCop. Most others are consulted by few receivers.
How do I get delisted?
Fix the cause, then use the operator's removal form. Requesting removal while the problem continues usually extends the listing.
How long do listings last?
SpamCop expires within about 24 hours of the last trap hit. Spamhaus listings stay until you request removal.
Can one bad campaign cause a listing?
Yes. A single send to a purchased list can hit enough traps in one afternoon to trigger one.