Some consider the blocklist to be the big, bad wolf—no one really knows how it looks, where it's found, or how dangerous it really is. Let’s go through all you need to know about this phenomenon.

A blocklist is a list of IPs​ and/or domains suspected of sending spam-like communication. These lists are managed by a Blocklist operator and ISPs use their service to understand and classify senders.

There are hundreds of blocklists, some major and some minor. The chances are high that all you as an sender of multiple emails will be moving on and off blocklists without it affecting marketing noticeably. ISPs don't always classify you as a spammer right away, they may give you the benefit of the doubt the first time. But if you stay on the blocklist they will end up classifying your emails as spam.


*ISP = Internet Service Provider or Inbox Service Provider. In this case, we use ISP as a collective term for the companies providing the email services e.g., Gmail Outlook Live and Yahoo.


Things that can lead to blocklisting

One bad email campaign doesn't mean you will end up on a blocklist. As always with emails, your sending pattern will play an important role here. It is important to have proper list hygiene and analyze your email marketing communication. Here are some factors that will affect your reputation as a sender and could lead toyou ending up on a blocklist:

  • Your emails are marked as spam by the recipients.
  • You have a high volume of send-outs and a low volume of engagement (open rate, amount of opens).
  • Your bounce rate is unusually high for a long time.
  • You are sending to spam traps.

How to know if you are on a blocklist

It can be tricky to see if you're blocklisted but one indication is a noticeable drop in open rates. A drop can also indicate that your emails have ended up in other folders than the recipients' inboxes. In this case the ISP might use their own set of rules to determine your sender reputation.

Another way of knowing if you are blocklisted is to use tools to monitor it. Please contact our support if you need more information about this.

How to get off a blocklist

As said, there are several types of blocklists and they vary in size, usage and impact. Most of the time you will be removed from the list automatically as your sender reputation increases. Some of the larger blocklist operators will have specific instructions on how to request a de-listing. In those cases you are required to act accordingly.

There are several tools you can use to monitor your listings—MX Toolbox (, HetrixTools ( to mention a couple.

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