A blocklist is a database that uses certain pieces of information to determine if an IP or domain is sending messages that could be considered spam. There are hundreds of blocklists out there, though only a handful of them are used widely enough to have a major impact on your email delivery rates.
Blocklists are tools that are used by ISPs to inform decisions on whether or not to place a message in the end user’s inbox. If an IP or domain is on a blocklist, ISPs that use that blocklist to inform inbox decisions will not accept mail from that IP. Most blocklists are dynamic, meaning that they will delist automatically after a given period of time or when the sender’s email statistics improve.
An IP will be included on a blocklist if certain criteria are met, and these criteria vary. Spamhaus, the world’s most well repudiated and widely used blocklist, has a great graphic that explains how their blocklist feeds information to ISPs and what happens from there.
While blocklists use a number of methods to determine whether or not an IP is sending messages that could be considered spam, two of the most common are spamtraps addresses and user feedback. Spamtraps are email addresses that are not meant to receive email. Some spamtraps were created by anti-spam professionals and were never meant to receive email, while others are simply old invalid addresses that have been repurposed. You can read more about spam traps here: What Is a Spamtrap and Why Do They Matter?. Blocklists also rely heavily on feedback from users that tell them that a particular IP or sender is sending spam.
We have to be strict about our Email Use and Anti-Spam Policy because in the rare event that one of our customers triggers a blocklist, any customer on the same IP range will be affected.
To check if an IP is blocklisted, use this tool.
If you have found that you are listed on a blocklist, please see this article, Blocklist Remediation, for a path to resolution.
To learn more about specific blacklists, please visit our guide to the top blacklists, Top Blocklists – What You Need to Know.