Resources Hub » Blog » Why Email Subject Line Testers Don’t Work

An email subject line can make or break your campaign. A solid subject line that resonates with subscribers will get your emails opened, read, and clicked. Meanwhile, a lackluster subject line will earn you a swipe to the trash bin—or even flagged for spam.

Sixty-nine percent of people will report an email as spam based only on the subject line.

You want to make sure your subject line is relevant, interesting, and engaging enough to get your subscribers to actually open your email. Plenty of people turn to a subject line tester to help them refine their openers.

However, an email subject line tester may not help as much as you would imagine.

In this post, we’ll explain how email subject line testers work and their limitations. We’ll also go over some tips for creating an amazing subject line and how you can run your own accurate tests.

Why were email subject line testers created?

Simply put, email subject line testers exist to help you write engaging subject lines. Most email service providers also offer free subject line testers built into their campaign builders.

When they first came out, subject line testers were effective tools for optimizing your copy’s length, word choice, emotion, and much more.

Most email subject line testers analyze your subject line and spit out a handful of suggestions for improvement. For beginners, these testers are still worth toying with to get an idea of what makes a subject line “good” and “bad.”

What’s wrong with using an email subject line tester?

In many cases, using an email subject line tester is useful for monitoring your character count or keeping an eye out for spammy words. When it comes to the actual copy, however, subject line testers are pretty unreliable indicators of how an email will actually perform.

General data doesn’t work for each industry or unique audience.

Now, this isn’t the fault of developers or the companies who make these testers. The problem is that you simply can’t predict how an email subject line will perform with unique people and audiences.

Subject line testers don’t ask you to select your industry or product and they don’t ask you to provide any information about your audience. Instead, they use data from general sources to gauge future performance.

Publishers and NGOs are probably the worst industries to use these testers.


Many marketing tools, like subject line testers, are designed for use with retail or e-commerce industries. Attention-grabbing words for Foreign Policy magazine or UNICEF emails would probably be quite boring or irrelevant for subscribers to Adidas, for example.

Take this for example. The email tested below had the subject line “What’s really happening between the US and Turkey?” It performed pretty well with a 28.1% open rate, a good indicator of the subject line’s quality.

However, when we run the subject line through this tester, it tells us the subject line needs a lot of work. If the creator had updated her copy based on the results of this email subject line tester, she would have wasted her time fixing something that wasn’t broken and could have even ended up with a lower open rate.

Source: Coschedule

They’re static and limited.

Email subject line testers are only as good as the information developers give them. Since marketing is a dynamic industry with trends that constantly evolve, this creates a problem.

For example, words that boost open rates in December probably won’t perform well in July.

Subject line testers do their job by analyzing your subject line and looking for certain words. They look for words the developers believe can improve open rates and they look for specific words that may decrease your open rate.

If you wanted to create an accurate subject line tester, it would have to use an algorithm based on real-time information about your own subscribers and industry. Otherwise, the suggestions about “good” and “bad” words are just too general to put to any good use.

How to write an amazing subject line

Testing certainly plays an important role in optimizing your email campaigns, but instead of relying on generic online tools, look at your own email campaigns to see what works for your specific industry and audience. You can even run A/B tests to see which subject line performs better before sending it out to your entire list.

Plus, you can use the 8Seconds tool within your Campaign Monitor emails to see your best results. 8Seconds will test hundreds of different combinations and optimize them in real-time for subscribers who haven’t opened the email yet.

Keep these tips below in mind as you write your email subject lines.

1. Personalize as much as possible.

Using your subscriber’s real first name in the subject line can boost your open rates by 26%. However, personalization is much more than just including your subscribers’ names: You can personalize the content of your subject lines, too.

Which types of subject lines have worked for the subscriber in the past? Which products or services have they shown interest in? What have they purchased from you before? Where do they live and what do they do for work?

Creating segments based on this data and then incorporating some of this information into your subject line formula will make the copy as relevant as possible.

2. Don’t send the same subject line to your entire list.

Break your subscriber list up into different segments and create unique subject lines for each group.

You can segment your list based on

  • Location
  • Browser
  • Email client
  • Age
  • Gender
  • Income level
  • Behavior
  • Parental status

It’s easier to write a relevant subject line to 50 people rather than 150 people.

3. Write for one person.

Too many marketers make the mistake of writing generic subject lines. Even if you’re sending a campaign to your entire subscriber list, imagine that you’re writing for one specific person.

Email is an intimate method of communication. It’s not cluttered with comments and people have voluntarily given you their information.

Radicati estimates that 347.3 billion emails will travel throughout the internet every day by 2023. Treating email like a personal conversation helps break up the clutter.

4. Keep subject lines short.

Most email apps will cut off your subject line after 30 or 40 characters—that’s not much space to get your point across.

Try to keep your subject lines as short as possible. Use data from your previous campaigns to check out which email clients your subscribers use. Do they tend to open your emails on Gmail for Chrome or their smartphone?

If someone opens your emails on a desktop, you have more room to express yourself. Use this information as you create your subject lines.

5. Know what works for your audience (and audience segments).

Each audience—and audience segment—is unique. Subject lines that perform well inside your industry probably won’t work for other fields.

Even within your own audience, different segments will respond better to different subject lines—it’s just human nature.

Instead of treating best practices as gospel, learn about what your audience likes.

6. Don’t settle for any filler.

Filler doesn’t have any place in your email copy and there’s absolutely no room for it in your subject line. With precious real estate in push notifications, you can’t afford to include any fluff that doesn’t directly relate to the point you’re trying to make.

Eliminating filler is easier said than done. In many cases, you don’t’ even realize you’re writing filler—it just sneaks in. After you write a subject line, analyze it to see if you can say the same thing in fewer words.

You could also ask someone else to look over and reword your subject line to make it more concise.

How to be your own subject line tester

We’ve spent a lot of time pointing out the problems with email subject line testers here. We’re not saying you shouldn’t test your subject lines – we’re just saying that technology doesn’t have everything figured out yet.

Instead, you’ll get much better results if you function as your own subject line tester.

A/B testing or split testing is much more effective because it tests subject lines among people who are actually receiving the emails.

After breaking your subscriber list into segments, create one campaign but write two unique subject lines. Send the campaign with one subject line to half of your subscribers and the other subject line to the other half.

Which email earned more opens? That’s the better subject line. You could also scroll through your past campaigns and look for patterns.

When you A/B test your own email subject lines, you can still run tests for all the different variables that online testers check.

  • Length
  • Power words
  • Questions
  • List numbers
  • Emojis
  • Emotion

Wrap up

While using an email subject line tester can help you catch any major red flags, the technology falls short in a lot of aspects.

  • Subject line testers are fed static information.
  • Subject line testers don’t account for unique audiences or different industries.
  • Start by designing an engaging subject line with best practices.
  • A/B or split test your own subject lines.

By understanding your audience, you can write the best subject lines and test them with the most accurate data.

Do you need some more help developing killer subject lines? Check out this post with six subject line tips to break through the clutter.

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This blog provides general information and discussion about email marketing and related subjects. The content provided in this blog ("Content”), should not be construed as and is not intended to constitute financial, legal or tax advice. You should seek the advice of professionals prior to acting upon any information contained in the Content. All Content is provided strictly “as is” and we make no warranty or representation of any kind regarding the Content.
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