Why Was My Email Blocked?

Derek Harding

3 minute read

Oftentimes, absolutely no one knows why your email got blocked or bulked, but that isn’t necessarily a problem.


The answer to the question, “Why was my email blocked?” has changed in the last 10 years. I’ve heard this question many times. It’s the first question anyone asks when they find out they have a deliverability issue. A decade ago, the answer was usually pretty straightforward. Blocklists, spamtraps, complaints, bounces, spam keywords—whichever the specific cause—it was generally fairly easy to figure out.

Today, though, the honest answer in many cases is, “I don’t know.”

This isn’t a popular answer. It’s not one that most deliverability people are comfortable giving, and it’s certainly not one clients want to hear, so it’s not usually the answer you’ll be given. Instead, possible, plausible, and likely causes are given. The truth is that not even the ISP’s own staff actually knows for sure why a given email was blocked.

A little while back, I had the pleasure of attending the Email Evolution Conference. The final panel was about deliverability and featured filtering and postmaster staff from Google, AOL, Microsoft, and Comcast—all smart, knowledgeable folks who are intimately familiar with their systems’ filtering. The representative from Google highlighted the situation most clearly when he explained that filtering decisions are based on thousands of signals (data points). These signals are combined together by machine learning algorithms that work at multiple levels. There’s filtering by sending IP addresses, comparison of messages for similarities, and also individual-level filtering. “One man’s spam is another man’s ham.” To make it even trickier for marketers, ISPs look at a range of signals into which marketers have no visibility.

Their view of how their users are interacting with messages is different from our view. They see some things we cannot while we see some things they cannot. The result for anyone trying to identify the cause of bulking or blocking is that they’re often trying to achieve the impossible.

This may sound hopeless, but it really is not. It just requires a change of approach. Consistent, successful delivery depends on providing mutual value to customers and marketers. The members of the ISP panel put it slightly differently but said the same thing. They challenged us to create interesting and relevant messages that their recipients will want to receive.

That’s great in theory, but we all know that business requirements and resource limitations often create a gap between what we’d like our campaigns to be and what they actually are. Even so, solid deliverability is achievable with effective campaign management.

What that means in practice is looking at our programs from start to finish—from opt-in and permission collection, through engagement and maintenance, to opt-out and ramp down for disengaged subscribers.

Here are the key elements.

  1. Infrastructure is table stakes. Get your technical ducks in a row. Authentication, forward and reverse DNS, properly functioning reply and unsubscribe mechanisms, feedback loop membership—if you don’t have these working, you’re starting at a disadvantage.
  2. List hygiene is integral. It’s not just a bolt-on afterthought. It must start at the beginning and run throughout your program. How you collect addresses, how you obtain and verify permission, how you ramp down and ultimately offboard the disinterested and dissatisfied all matters. Once you have deliverability due to poor hygiene, it’s too late. At that point, the cure may be worse than the disease.
  3. Deliver value. There are many who question the importance of engagement, especially since the metrics of marketers and ISPs are so different. Engagement, though, is a measure of the value you’re delivering. ISPs are working hard to ensure that the email users receive in their inboxes is the email they want in their inboxes. This applies whether you’re an offshore pharmacy spammer or a Fortune 500. Legitimacy, especially your perception of your legitimacy, matters little. How your subscribers respond to your messaging, with anger and complaints, with indifference or with positive response, affects your delivery. It affects it to that individual and ultimately to your entire list.

In short, my takeaway from the EEC deliverability session was that ISP filters are really, really complicated and based on signals we can’t measure. These systems, though, are designed to ultimately reward senders who honor, respect, and value their subscribers.



Need assistance taking your email program to the next level? Reach out to the email experts at Trendline today.



Trendline Interactive

Ready to send better messages?

About the Author(s)

Derek Harding

MarTech before MarTech was a thing. Founder and architect behind Innovyx, one of the first email service providers. Author, contributor, trainer, and conference speaker. BS w/Honors in IT from DeMontfort University in England. In his spare time a musician and a sailor. Owner of a messed up accent; Bark > Bite; Looks 6’2” on Zoom calls. Follow Derek on LinkedIn

Let's Take This to the Inbox

Sign up for our news, resources and updates. The inbox is our favorite place after all. We’ll make sure it’s worth it. (You can unsubscribe at any time, but you probably already knew that.)