Here's an email test I'd love to run:
- Retroactively look at every subject line from the past few years and group them by content type: discount, announcement, urgency, numbers in the subject line, and so on.
- Look at which users are most likely to respond to which message type, and group your users accordingly.
- Test sending users only their favorite message type for a few months.
- Measure revenue and email engagement, and also how much your email costs declined.
- If it works, test layering paid media retargeting on top.
What I've seen is that companies tend to find that sending more emails means more revenue. But over time, people stop opening, stop clicking, and stop buying, because there's nothing interesting in their inbox.
If you hook people with a subject line you know they already like and respond to, they'll seek your emails out. And you save on sending emails that do nothing for certain users.