Testing AI Tools to Write Email Subject Lines

AI can help you write better subject lines, or can it? We looked at a few popular tools to see what they can produce and the results might surprise you.

Subject lines can be hard — are they easier with help from AI?

We’ve said it before and we’ll say it again — getting your subject line right can be hard.

Part art, part science and, it seems, part luck, subject lines are a very important component of any cold email. Coming up with a great subject line is the key to lots of good things and the cause of lots of bad things when it goes wrong.

For such a small piece of real estate, subject lines have a huge impact on any email campaign. When they’re good, they can boost open rates, conversion rates and encourage recipient behavior that scores points with spam filters (the good kind of points). They’re also another opportunity to personalize your email.

When your subject line is meh or just plain bad, it means the content inside your email won’t be seen by as many people, no matter how good it is, with predictable results for your bottom line.

Tips for writing effective email subject lines with AI tools

Following these few tips is usually enough to pass the subject line test:

  • Keep it short (and sweet!). You only have so many characters, this is not the place for long, extended messaging.
  • Make it relevant. This is also not the place for vague, clickbait-y attention grabbers that have nothing to do with what’s inside.
  • Use personalization. If everyone you send the message to sees the exact same subject line, you’re doing it wrong.
  • Test different variations, go with what works. Subject lines are perfect for A/B testing, where you get proof of which of two or more variations better connects with recipients.

People don’t respond to what they don’t read, and good subject lines get more people to read what’s there. It’s that simple.

Adding AI to the mix

Like so many other things, even email subject lines are getting a boost from Artificial Intelligence. Suddenly there are tools that apply AI to your subject lines and even write them for you.

AI is already well-established as a tool for writing the content of emails and its role will undoubtedly continue to grow. But subject lines are a different niche, with their own rules and challenges.

That’s why we decided to test some AI tools that write and evaluate subject lines. Are they good enough? Are they great? Or do they, um, need some time before they reach that status?

Let’s go through some of the popular subject line generators and see what they come up with!


It’s right there in the name — Copymatic offers tons of different kinds of copy, from blog posts to FAQs to meta descriptions and everything in between. Email subject lines is just one of the many, many options it offers. When you sign up for a free trial, you are given 1,500 credits to spend as you like. I spent mine here, of course:

Once inside the Email Subject Lines module, you are prompted to provide information that their AI needs.

The list of languages is fairly impressive — around 30 — and includes some you wouldn’t expect, so keep this in mind the next time you need to send cold emails in Indonesian.

As for creativity, you can choose between ‘Regular’ and ‘High’. This might seem like an obvious choice, like choosing “extra strength” aspirin over “regular”, but we’ll compare the different outputs in a moment.

Tone of voice is next and the dropdown menu contains an option I would have never guessed in a hundred years:

You have to wonder what kind of feedback they got from customers in order to add this option (My emails just aren’t childish enough…).

Anyway, next up are audience, product name and product description. It’s odd and frustrating that the only option is to write about a “product”. No option for an email asking for a phone call or meeting or anything else. With Copymatic you can get help on your subject lines for emails about a product and that’s it. Maybe we can cheat a little and describe a service instead? We’ll see.

I wrote that the ‘product’ was a sales training course and for the ‘product description’, I wrote this:

A sales training course that improves the skills of your sales staff and results in more conversions.

Here’s what I got in return:

When I switched the ‘Creativity’ to ‘high’, the results looked like this:

Yes, “Here’s something you don’t need” was returned as a subject line I should use. I don’t even know what to say.

Comments on Copymatic

The good:

  • Easy & straightforward to use
  • UX leads you through what you need to do
  • Some of the results were…ok

The bad:

  • None of the subject lines produced was something that I couldn’t come up with myself
  • Some of the subject lines, like “The #1 sales training course”, are both spammy and generic at the same time
  • You can only get subject lines for one kind of email, promoting a ‘product’

The ugly:

  • “Here’s something you don’t need”? This makes me think that Copymatic is something I don’t need.
  • “Improve Your Sales Team’s Success with These Quotes”? Am I selling a sales training course or a book of quotations? Here’s a quote for you — “Keep looking for another AI subject line tool.”

Salesblink generates email subject lines and their website lets you get right to it. You get five free subject lines that are “battle-tested for high open rates.” I used the same inputs as with Copymatic to keep it fair:

It’s good that I have this screenshot to prove what the results were, because otherwise you wouldn’t believe me:

I really want to know what “battle” these subject lines survived and where they got those open rate numbers from but I’ll just have to somehow go on without this information. In what universe does “Huge fan” get an 80% open rate? Or even an 0.80% open rate?

Comments on Salesblink

The good:

  • Some comic relief while preparing this post.

The bad:

  • So much bad, so little time. I’ll let those results speak for themselves. “Enough said!”

The ugly:

  • “Can you point me in the right direction?” Yes, I can — the direction that takes you away from Salesblink for your subject line needs.


Encharge’s website also lets you get started right away without any registration or other formalities. Here’s the input I shared on their main page:

The only drop-down menu is for the Tone, and the options are interesting:

A few too many, I think, and most of us would question the difference between “enthusiastic”, “inspirational”, “joyful” and “passionate” in this context but I will admit to being curious about “awestruck”.

Anyway, here are the “friendly” results I got:

Better than what we’ve seen so far from other sources but still not great. Why did they decide that I want to offer a 30% discount on my course? And why are they making promises like “increase sales by 30% in 30 days” for me? Shouldn’t they have clarified that with me first?

I just couldn’t resist the temptation to see what “awestruck” subject lines look like, so after changing my preference in the Tone menu, here’s what I got:

Las Vegas? What webinar? Where did these come from? I’m awestruck at how unimpressive this is.

And I also couldn’t leave encharge.io behind without seeing what their idea of “Humorous” is, so I changed the Tone again. Here are the results:

Let’s just say that we have different ideas about what’s funny and what isn’t. Anyone know what “Nussle-Flakes” are?

Comments on encharge.io

The good:

  • Unlike previous names on this list, encharge.io didn’t return anything really embarrassing. Except “You Don’t Know Sales like Rocky (or Do You?). Kind of embarrassing.
  • At least some of them are actually useful as subject lines.
  • Super-easy to use.

The bad:

  • Too many of the subject lines are nonsensical or just plain weird.
  • The huge choice of Tones only seems to result in different kinds of weird.
  • There’s nothing here that you couldn’t think of yourself.

The ugly:

  • Nussle-Flakes


Next up is Storylab.ai and their general tool that creates everything from story ideas to descriptions to ad copy and more. Here’s where we will start:

First we choose from a list of types of email. For us, it’s ‘Cold Email’:

Next we say a few words about the point of our email. We’ll use the same text as before:

For Writing Style, let’s keep it Professional (although ‘Sassy’ looks tempting):

It’s at this point, after you’ve entered all the information you need to get a subject line, that Storylab.io decides to show you that they have the worst UX ever by triggering this pop-up:

Yes, now they want you to sign up so you can get your results, complete with setting up an account and everything. Ok, fine. We’ve come this far so let’s finish. After setting up an account, Storylab.ai decides to make an even worse impression with this:

You want to talk about money now? Just give me my free results! After jumping over the last hurdle, I got this message:

About two minutes after that, I finally got the results:

As badly as I wanted to run away screaming from Storylab.io, I just had to see what ‘Sassy’ emails look like. I changed the tone and got this:

Now I understand. ‘Sassy’ apparently means ‘with an exclamation mark at the end’. Got it. Thanks, Storylab!

Comments on Storylab.io

The good:

  • I’ll have to get back to you on this

The bad:

  • Worst. UX. Ever.
  • There doesn’t seem to be any Artificial Intelligence, or any intelligence at all, behind these subject lines.
  • Zero subject lines that anyone who speaks English can’t come up with themselves.

The ugly:

  • We’ll come back to this another day in a 10-part series. No time for it now.

The bottom line on writing subject lines with AI tools

I’ll be honest with you. When we first considered AI-powered subject lines as a topic, we had higher expectations. We thought that some of the tools that create such impressive longer-form texts could do the same with the unique demands of subject lines.

We were wrong. Seriously, totally, wrong.

As you can see even from the limited selection above, AI still has a long way to go when it comes to helping with subject lines for emails. Yes, we could have provided more or different input but, based on what we see here, there’s no reason to think the results would be very different.

We’re still waiting for an AI tool that can write subject lines that make you think “I would have never thought of that myself.” And isn’t that the real test of how good they are (or aren’t)?

My takeaway from this is that there is much more of a human factor in subject lines. Like I said at the start, subject lines are part art and machines aren’t very good at art. Subject lines are supposed to appeal to us as humans and maybe other humans will always be better at that.

Another lesson here is that you shouldn’t “outsource” all of your copy-related needs to AI. Not yet, anyway. Look at some of the terrible output above — would you blindly trust any of these tools and just use whatever they came up with? Of course not. You always need to be involved in all texts before you send them out, rewriting, editing or changing things so that they sound like normal human speech.

In other words, beware of the Nussle-Flakes!


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