This week we cover one of the most feared words in email: blacklists. If your sending IP or domain appears on a major blocklist, your email deliverability can collapse overnight.
This Week’s Lesson
Email blacklists (also called Real-time Blackhole Lists or RBLs) are databases of IP addresses and domains known to send spam, malware, or other malicious content. Mail servers query these lists before delivering incoming mail.
There are over 100 public blacklists, but a handful carry the most weight: Spamhaus (SBL, XBL, PBL), SpamCop, SORBS, Barracuda, and MX Toolbox's own composite list.
How do you get blacklisted? Common causes include: high spam complaint rates (recipients clicking 'Mark as Spam'), sending to honeypot email addresses maintained by blacklist operators, emailing purchased or unverified lists, and sudden spikes in sending volume.
Once you're on a blacklist, the remediation process varies by provider. Spamhaus requires you to identify and fix the root cause before submitting a delisting request. Many lists auto-expire after a few days or weeks if sending behavior improves.
The best strategy is prevention: maintain clean lists, monitor complaint rates, authenticate properly, and keep a consistent sending cadence. Sudden volume spikes are a blacklisting trigger even for legitimate senders.
You can check whether your domain or IP is currently blacklisted at mxtoolbox.com/blacklists.aspx.