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11 Email Subject Line Tester Tools to Increase Your Engagement

Marketing and sales have embraced technology due to the need to connect better with prospects. Subject line testers are one of the new breeds of tools that have emerged on the frontline of email marketing and managed to turn the art of writing an email into a science. 

Nothing displays more effort like a good subject line. When we talk about emails, subject lines can make or break your campaigns.

Did you know that 43% of email recipients check out the subject line before they categorize it as spam or not? That’s why it’s vital to consider your subject lines carefully and avoid the dreaded spam folder.

But how do you do that? 


That’s where everything begins, at least. You can increase your open rate by 2.6% by adding the recipient’s name in the subject line. 

But getting high open rates is not as simple as adding one personalization tag. There’s a lot more involved in creating a successful subject line. 

That’s where subject line testers come to the rescue. These tools use public data, deliverability best practices, proven templates, and the developers’ vast experience to rank your subject lines and suggest improvements.

This post will run through some of the most popular subject line tester tools that will help increase your deliverability and open rate. 

What is an email subject line tester tool?

An email subject line tester tool is a simple-to-use yet powerful tool that helps you check if your subject lines have words that are considered spammy and provides tips on how to make your subject lines more enticing. These tools check for various parameters like sentiment, reading level, spam words, and personalization. They usually return a score, based on which you can decide to use the subject line as it is or make adjustments. 

But do you really need a good subject line? 

The subject line of your email is the most critical thing you will write in a message. Why?

More people will read your subject line than any of the inside text. They also utilize that 3-second first impression to identify whether they will even click on your email or not. Remember that you only have one chance, and it could determine the fate of your whole marketing campaign. 

In fact, 35% of email recipients open an email based only on the subject lines. The subject is the headline of your email, and like blog post headlines, that is where it lives or dies.

You should not be surprised knowing that email marketers spend most of their time looking for many ways to make their subject lines more enticing. 

The best email subject line tester tools

Here are some of the best subject line tester tools that will help make your email subject lines more irresistible: 

1. Spam Check by Postmark

Do your emails end up in spam? Then it would help if you use a spam tester such as Spam Check by Postmark

This tool tests your subject line and your entire email body to determine if your message will be categorized as spam by email clients. It will also offer basic details on why your message has the score and how you can improve it.

Even though Spam Check only checks text and not images, it is still a good tool. That’s especially true if you search for an easy, no-registration option to test your emails before hitting the Send button. 

Just copy and paste the subject line, email message, and header text, and click the “Check Score” button. Spam Check will tell you what words or other factors will trigger spam filters.

2. Mizy 

Automizy built Mizy generator to provide attractive and unique email subject line suggestions. The main job of this subject line generator is to ensure your emails get read. After all, nothing is worse than investing money and time into your marketing efforts, only to have your emails never opened by your audience. 

Mizy is an AI tool that offers you insights based on real-life data. It ranks subject lines by comparing them against other campaigns from their dataset. By using machine learning, it compares the performances of such campaigns and recommends click-worthy subject lines to enhance your newsletters’ open rates. It accesses a database of emails to evaluate the open and click-through rates. 

The tool produces a performance-based rating of your subject line and rates that on a 0 to 100 scale. 


Having a hard time choosing between two or more subject lines? Use the power of A/B testing and make the right decision based on real data. helps you compare two versions of an email and determine which one leads to the most clicks. 

Source: How to send A/B testing campaigns

The A/B testing feature enables you to easily test different variations of a single email in your campaign to identify the most successful one. 

Experiment by adding small changes to the design, call to action, copywriting style, subject line, and other aspects of your email. Compare results and know which works best for your audience.

Read more: What is A/B Testing in Email Marketing? The Essential Guide

4. Test Subject by Zurb

Design and branding solutions company Zurb provides the Test Subject tool to assess subject line appearance on mobile devices. Many people read emails on their mobile phones, and that number is bound to rise with the increase of mobe device adoption.

Zurb realized that email previews and subjects are truncated on such devices. A poor display of a subject line might put off the reader. Further, inappropriate phrases and incomplete sentences might push all the efforts down the drain.

Test Subject is a fantastic tool that addresses that concern. It guarantees your email headline is always compatible with all the newest smartphone displays available on the market. The email subject line tester is simple to use and helps you make mobile-friendly headlines for your newsletters. 

The tool also provides a preview of the sender’s name, headline, and preheader text. The preview features help guarantee your newsletter’s subject line appears in your readers’ inboxes. 

5. Omnisend

The Omnisend subject line tester gives you a percentage score on how effective your subject line would be. It also provides tips on how to improve your subject lines, such as the recommended length and wording. It suggests effective words and highly words should be replaced with safer alternatives.

It also will point to risky parts of your subject line that could lead to getting your email marked s spam.

On top of that, it also gives you a preview of what your subject line would look like on mobile.

6. Email Subject Line Grader

This simple email subject line tester by Net Atlantic provides a short description of the subject line type you are using, word count, details the character, and evaluates the word mixture and balance. It is that simple.

The evaluation is based on character count, word count, power words, and word mixture and balance. Simply enter your subject line in the field and click Go.

7. Send Check It 

Checking in email subject lines with Send Check It is a breeze. It’s an awesome, simple-to-use email subject line tester that offers similar insights as the other tools provided, including reading level, scanability, and length in both words and characters. 

Also, it provides a plethora of additional information, such as: 

  • a scan of other elements like personalization and use of emojis
  • analysis of capitalization, punctuation, and other word choices
  • possibly spammy words
  • a comprehensive analysis of sentiment analysis
  • a preview of how the subject line reads on mobile and desktop devices 

Send Check It offers you an inclusive view of all aspects of your email. That makes it a must-have for every marketer out there. 

8. CoSchedule

The email subject line tester of CoSchedule offers a comprehensive assessment along with actionable tips to help you craft effective subject lines.

The guide will help you enhance the subject lines when it comes to word placement, structure, and grammar. Given those qualities, CoSchedule is the ideal tool for marketers seeking to master the art of writing good subject lines. 

On top of that, you can effortlessly compare previous headlines. You can test several variations and record how higher-scored subject lines fair over lower-scored lines for overall engagement and open rates. 

9. Mailshake

The Email Copy Analyzer evaluates your copy for spammy words and readability. It checks your email for links, words that trigger spam filters, and length. 

By copy, we don’t simply mean the subject line but the email body as well.

It offers specific suggestions to change your copy until readability and deliverability are enhanced. 

The tool recommends personalized subject lines with 30 or fewer characters to accomplish an above-average open rate. It also helps you take advantage of urgency through the email subject while avoiding being clickbaity. 

This email subject line tester is an excellent tool to help you craft compelling headlines with prompt guidance and is a favorite among salespeople, marketers, and entrepreneurs. 

10. Hemingway App

The Hemingway App was designed for writers who want to streamline their copy. It does this by thoroughly evaluating whether a layman can understand what you are talking about. It is extremely useful if you plan to edit the email’s body. 

To get results, just copy and paste the email content into the tool, and you will receive a readability grade. From there, you will realize how often you have used passive voice, the number of adverbs, and the most complicated sentences to read.

11. is essential for every marketing team. This subject line tester assesses any subject line based on email marketing best practices and offers you an instant score between 0 and 100.

More important than the score itself is the feedback provided by the tool. For instance, let’s say your subject line fails to attract users. In that case, the tool will deduct points for not using urgency.

It’s worth mentioning that checking in email subject linсe with means you receive actionable advice right away. You can continue changing your subject line until you achieve your desired results. 

Bonus tip 1: Re-send your email with a different subject line

One of the easiest ways to increase the open rate of your email campaigns is to send the same email with a different subject line to all un-opens. We’ve covered the easiest and most flexible way to do this In our in-depth guide on the topic.

Bonus tip 2: Get subject line inspiration with the Free AI Subject Line Generator

Struggling to come up with a good subject line to start with? Our Free AI Subject Line Generator can spill dozens of unique subject lines based on a topic and tone. Just enter the topic of your email and the preferred tone, and the tool will generate dozens of subject line ideas for you. Then, you can take these ideas, adjust them, and run them through one of the subject line testers for a complete assessment. 

Final thoughts

If you are not sure how to get started with evaluating your subject lines, simply give one of these tools a try.

By making small changes to the words and phrases you use, you can fully realize the benefits of a well-crafted subject line — higher open rates and a more engaged audience. As you get into your creative space and send a few email campaigns, you will determine what works best for your contacts. 

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