A framework that helps to discover a meaningful email KPI
I get it. You're obviously not doing email marketing because it's a new fashionable thing to do. You want an impact with your work. Even more than that you want to make sure your work is leading towards that business improvement. You'd love to hear a definitive answer on what is the best KPI. Let me tell you right away. There's no best answer :(
No Best Answer
It might be wise to see what other marketing managers prefer as their KPIs. But copying others most likely won't make a difference for you. Different businesses have different aims and that leads to different strategies and as a result - different KPIs.
Dumb KPI → Dumb actions
Here's an example of how a wrong KPI can lead to not-so-smart decisions. Let's say your task is to improve open rate. That is very simple! Just drop 50% of your subscribers that haven't opened your email in the past year. Done, now a higher percentage of the subscribers open your emails. Do it a few times and your open rate is around 90%.
Put your KPI thinking into reverse
Instead you should reverse your workflow. Figure out what drives your business the most, or at least one aspect you can work with. Then figure out how your email campaigns can affect this business driver and find a KPI that will support this strategy.
Let's imagine you help an airline company with their email marketing. The first step would be to figure out what drives their sales. Some of it comes from vacation travel. There are multiple ways to affect these sales. How about blogs about exotic destinations? If you like this strategy, your KPI can be readership growth via emails. Of course, you can never be surely right, but at least this way provides you a way of thinking about KPIs and business issues. Plus, experiments like this can lead you to better insights from further data.