You're sending emails normally, but they're not arriving in inboxes — or you're getting bounce messages with cryptic error codes. One of the most common causes is that your sending IP address has been added to an email blacklist.
An email blacklist (also called an RBL — Real-time Blackhole List, or DNSBL — DNS-based Blacklist) is a database of IP addresses that have been flagged for sending spam, hosting malware, or other abusive behaviour. Mail servers around the world query these lists in real-time and use them to filter incoming connections.
There are dozens of blacklists operated by different organisations, and each has its own criteria for listing and delisting IPs. The most influential include:
dul (dynamic IP) list catches residential IPs trying to send directly.There are several common reasons an IP ends up blacklisted:
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Use Fred's blacklist checker to check your mail server's IP against 40+ blacklists at once. You'll need your sending IP — this is the IP of your mail server, not your workstation. You can find it in the email headers of a sent message by looking for the Received: headers.
Each blacklist has its own delisting process. Here's how the major ones work:
Spamhaus — Visit check.spamhaus.org, enter your IP, and follow the delisting link. For SBL listings (active spam source), you must demonstrate the issue is resolved. PBL listings can usually be removed immediately via their self-service form if you're a legitimate mail server.
SpamCop — SpamCop listings expire automatically after 24 hours of no new reports. The best approach is simply to stop sending spam and wait it out.
Barracuda — Submit a removal request at barracudacentral.org/rbl/removal-request. They may ask for more information about your sending practices.
SORBS — Visit sorbs.net and use their delisting portal. Different sub-lists have different requirements.
Don't submit a delisting request until you've fixed the underlying problem. Blacklist operators can see if you've been re-listed after a previous removal, and repeat requests without remediation may result in permanent listing.