Email blacklists — also called DNS-based Blackhole Lists (DNSBLs) — are databases of domains and IP addresses known to send spam. When an email arrives, receiving servers check the sending domain and IP against hundreds of blacklists. If your domain is listed, the email is rejected or sent directly to spam. Blacklisting is a common cause of sudden cold email deliverability collapse — a campaign that was performing well stops getting replies overnight because the sending domain was listed.
How to Check If Your Domain Is Blacklisted
The fastest tool for blacklist checking is MXToolbox's Blacklist Check at mxtoolbox.com/blacklists.aspx. Enter your sending domain and it checks against 100+ blacklists simultaneously in under 30 seconds. Google Postmaster Tools provides reputation data for domains sending to Gmail. Microsoft's SNDS (Smart Network Data Services) tracks reputation for Outlook-destined mail. Check all three for a complete picture.
Which Blacklists Matter Most
The highest-impact blacklists for cold email senders are Spamhaus (SBL, XBL, PBL), Barracuda, SORBS, and SpamCop. A listing on Spamhaus will block delivery to a significant percentage of enterprise email inboxes. Barracuda is common in small business and mid-market environments. A clean check on MXToolbox covering these providers confirms your domain is not blocked at the infrastructure level.
What to Do If Your Domain Is Listed
If the domain is listed, each blacklist has a delisting request process — typically requiring you to identify why you were listed and confirm the issue is corrected. Spamhaus delisting requires addressing the specific spam source. For cold email specifically, the most common listing cause is sending to invalid addresses at high volume. The correct long-term approach is prevention: email verification with Hunter.io before every send, bounce rate management below 2 percent, and proper sending infrastructure. Omni monitors blacklist status continuously for every domain in its outbound client campaigns.