I was digging around in my email marketing analytics recently and noticed something interesting: the decay in open rates seemed to be much higher than I remember it being. If you’re unfamiliar with the term, email open rate decay is how quickly your audience opens up your email. For example, if you sent an email marketing message on day 1, there would still be people opening it on day 2, fewer on day 3, fewer still on day 4, etc. Each day that passes, fewer and fewer people open your email. The rate of decay tells you how long the shelf-life of your email is: the faster the rate of decay, the more people who read it sooner rather than later.

Take a look at what my day 2-4 rates of decay look like between 2012 and 2013 for the same period of time, January 1 – June 15:


Big disclaimer: this is a sample set of 1, meaning that this is sampled from my personal newsletter. It is not in any way a representative sample of all email programs or all email lists, nor should it be interpreted as such. My email data is unique to my list only, and if you want to know what’s happening with your list, you’ll need to do your own analysis.

What I’m seeing is that my email list has become more immediate, more in the moment. Fewer people than ever read my emails on days 2-4 than ever before. What’s more, I see in my data that the number of people who open on day 1 has increased proportionally.

What could be causing this? I have a theory: more of my audience is on mobile devices. Let’s find out. First, we check the web analytics to see what email marketing traffic is coming from mobile.


Sure enough, my mobile web traffic coming from email marketing campaigns is up almost 40% year over year. That’s a big increase in the number of people reading and clicking through (tapping through?) on their emails they get from me.

Next, let’s look in the actual data from the WhatCounts Publicaster platform by browser counts. Here are the devices people used who opened my emails from June 2012:


And here’s the same mailing list from emails in June 2013:


The number of people opening on a mobile device went up 51.4% year over year. That’s an even greater number than the people tapping/clicking through to my website from email. In fact, the majority of the people who opened my emails in June 2013 are now doing so on a mobile device.

That explains much of the 0-day behavior and the rapidly increasing email open decay rate: when you can check your email frequently on a mobile device, you do so and you open email marketing messages on the day that you get them.

I would strongly encourage you to run this kind of analysis on your own email marketing data if you have it. You might be surprised at just how short the half-life of your email marketing messages is.

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