What is a blocklist?
A blocklist is a real-time collection of senders that are thought to be sources of spam or other types of email abuse. Blocklist providers may utilize a combination of spam traps, spam complaints, and other proprietary data sources as criteria for adding a sender to a blocklist. Inbox providers often maintain their own proprietary blocklists, using them in combination with independent, third-party blocklists.
The two types of blocklists
Generally speaking, there are two types of blocklists: IP-based and domain-based.
An IP-based blocklist is a real-time collection of sending IPs known, or suspected, to be sources of spam and other unwanted email. "IP" refers to IP address, which is a number that uniquely identifies any device connected to the internet. "IP" stands for "Internet Protocol." Similar to how a street address helps people find buildings, an IP Address helps computers find each other on the internet. Sending IPs can indicate who or what device sent a particular email communication.
In some cases, multiple "senders" may utilize the same IP address(es), as is the case when using your ESPs shared IPs, rather than dedicated ones.
A domain-based blocklist, in contrast, includes only internet domains and subdomains (e.g. litmus.com and mail.litmus.com) instead of IP addresses. Since a domain could send mail from a variety of IP addresses (for example, if you use multiple ESPs), landing on a domain-based blocklist can have even wider-reaching effects on deliverability. It can even impact mail sent by an organization's own employees, since those email addresses typically use the same domain.
What happens if I land on a blocklist?
The impact a blocklist has on your email delivery can vary depending on the list. To help you prioritize your efforts, we’ve included an impact flag, which indicates how damaging a listing could be for your email delivery.
A high impact listing indicates a widely used blocklist that will cause your email to be blocked at one or more major email providers. A low impact listing, by contrast, may impact your overall email delivery, but most likely on a lower scale.
How does Litmus help?
Litmus scans your sending domain and IP domain against various blocklists, indicating if you are listed at time of send. Not all blocklists carry the same level of influence or adoption by ISPs. Litmus will alert you based on the level of importance or possible impact of a backlisting.
What else can I do?
If you have found yourself on a blocklist, the most common reason is a bad source of subscriber data, user complaints, or hitting a spam trap.
Spam traps are commonly used by blocklist providers to catch malicious senders, but often, legitimate senders with poor data hygiene or acquisition practices get caught in a spam trap. Learn more by reading our guide to spam traps and how to avoid them.