What My A/B Tests Taught Me About Email Subject Lines (And What Still Works)
Let’s talk subject lines: one of the simplest ways to improve your email open rates.
I'm probably not the first person to say this but my goal for subject lines is always the same: make them irrelevant.
I want people to see my name in their inbox and open it even if the subject line is "asdjkac krw." (Note to self: try this one??)
Why Subject Lines Still Matter for Email Open Rates
To get to that level with someone, I have to get them to open the emails I send before they know me. Then they can decide if they're interested enough to open more of them.
And the best way to get people to open your emails is to write subject lines that make them curious about what's inside. It actually is that simple.
But knowing exactly how to create curiosity? Oof. Definitely not as simple.
That's where A/B testing email subject lines can help. You can look at what your audience is opening and what they're skipping. And often you get a clearer picture of what drives people's curiosity.
A/B Test #1
In my most recent email, I tested 2 subject lines:
As you can see from the trophy icon, B was the winner here. A was opened by 40.91% of the test audience and B was opened by 46.59%. That's a significant difference, right?
My guess: More people opened B because it sounded personal; it seemed like it might have been just to them and not to the whole list. Personalization has been a subject line best practice forever — apparently still true.
A/B Test #2
The gap was even bigger with these two! Only 35.59% opened A and 61.02% opened B.
My guess: Numbers in subject lines STILL WORK. Apparently it never gets old? It could also be that the "hiding" emoji isn't as fun as the clock. But I bet it's the number.
A/B Test #3
These subject lines are so different that it's hard to even compare them. But the numbers are interesting: 65.45% of the test audience opened A and only 46.67% opened B.
My guess: I think A was the winner because it opened a curiosity loop. Adding "(pt.1)" suggests a part 2, right? It says, “Wait. There’s more to this story…” Our brains LOVE dots that need to be connected!
The Takeaway: What Still Works in 2025 (and 2026!)
I don't always A/B test my subject lines but I'm going to start doing it more often. So far what I'm finding interesting is that the things that worked 5 years ago still work today: personalization, numbers, and an open loop. With social media algorithms throwing us new curveballs every day, it's kind of comforting that email subject line best practices have barely budged in years. Email is still… emailing. Cheers to that!