10 Best Email Subject Line Tester Tools To Boost Open Rates

List and comparison of best Email Subject Line Tester Tools, best practices for subject line testing so you can choose the best tool to achieve the highest email open rates:

You’ve probably put in a lot of time and effort into content creation for your email campaign. But how do you ensure that recipients actually open and read your email?

The success of any email marketing campaign depends on an email’s open rate, which, in turn, depends on the email subject line.

Email Subject Line Tester Tools

Subject Line Tester Tools

In this article, we will try to understand the importance of email subject line testing and some tips on devising strong subject lines. Get an overview of split testing subject lines. We will also review the top 10 email subject line tester tools, which will help in achieving high email open rates.

But first, what is email subject line testing?

Email subject line testing is using a variety of strategies and/or tools, to find out which email subject lines achieve the highest email open rates.

Fact Check: 69% of email recipients mark emails as spam based on the subject line alone. Research suggests that subject lines containing between six to ten words have the highest email open rate at 21%, and those with the recipient’s first name achieved an email open rate of 18.3%.

The image below shows some of the latest statistics and trends related to email subject lines:

latest statistics and trends related to email subject lines

[image source]

Pro-Tip: The best way to carry out subject line testing is to test your subject line using two different tools. You will find the best subject line somewhere between the two results.

Frequently Asked Questions

Q #1) What is an email subject line?

Answer: Itis like an introduction that informs the reader about the aim of the email. It appears beside the sender’s name and is the first thing recipients see when they look at their email inbox. The purpose of a good email subject line is to get the recipient to open the email. Usually, an email subject line offers some information about the contents of the email.

Q #2) What is email subject line testing?

Answer: This is a way of learning just how effective the subject line of your email is. Subject line testing involves sending the same email with different subject lines to small sections of your contact list, see which group gets the highest email open rate, then send the most successful subject line out to everyone else.

Q #3) How do you ‘test’ email subject lines?

Answer: This can be as complicated or as simple as required. The most basic form of subject line testing is split, or A/B testing, which basically involves comparing two subject lines to find out which one achieved a higher open- or click-rate, or a greater number of conversions.

Q #4) Why is email subject line testing important?

Answer: A subject line is the first thing email recipients look at in their inbox when they’re thinking about whether to open an email or send it to the bin. It is important that your email subject line is carefully crafted, so recipients don’t mark it as spam.

But how will you know if your carefully crafted subject line is really effective? This is where testing comes in. Testing your subject line is essential in order to ensure your efforts don’t go waste, and your email doesn’t end up in the spam or junk folder.

Q #5) What should you not put in the subject line of an email?

Answer: Avoid putting special characters in a subject line, as they’re likely to trigger spam filters. Also, don’t use ALL CAPS, it’ll seem like you’re shouting at the reader, and you’ll end up pushing them to click delete.

Q #6) What is the ideal length of an email subject line?

Answer: Given that most emails are read on phones nowadays, the ideal length of an email subject line is between 28-39 characters. This ensures your message is not truncated.

Q #7) What is a cold email?

Answer: A cold email is sent to someone with whom the sender has had no previous contact. An effective way to use cold emails is to personalize them in order to connect with likely customers.

Read on to learn more about the email subject line best practices, followed by our review of the top 10 email subject line tester tools that you can use to boost open rates.

Email Subject Line Best Practices

#1) Devising a strong email subject line

The perfect subject line can hook your recipient and get them to open your email, click on a link in your email, or even lead to a conversion.

Here are some best practices that we have compiled to help you craft strong subject lines:

  • Personalize it: Personalize the subject line by using the recipients’ location. Adding this personal touch will make the email seem more relevant to recipients and boost its open rate. Segmenting audiences further increases the effectiveness of personalized subject lines, as they can be customized for each group.
  • Ask a Question: According to email market research, subject lines framed as questions have a higher email open rate than those in the form of statements. Asking a question engages the reader, piques their curiosity, and invites them to open the email.
  • Say More with Less: Keep the subject line short. In fact, subject lines with 28-39 characters achieve the highest email open rates, because more people read emails on phones, and if the subject line is too long, they won’t even see all of it.
  • Include numbers: Subject lines with numbers get more clicks than those without. Numbers stand out, are easier to read, and add a specific focus to your message. They can also be used to add a sense of urgency in the form of deadlines or discounts.
  • Honesty is the Best Policy: It is not a good idea to lie or mislead with subject lines. Even if it boosts the open rates for that particular email, it is likely to annoy recipients, who might delete any future emails without even opening them or simply unsubscribe.
  • Avoid Spammy Words: Emails with certain words in the subject line will get sent straight to the junk folder. This is because of spam detectors that filter out emails based on keywords. You can increase the chances of your email being opened by simply avoiding ‘spammy’ words.
  • Make it Worthwhile: Write a subject line that suggests the reader will find something of value in the contents of the email, something they need, or a solution to their problem. Subject lines that offer something of value, for example, a discount, or an e-book, significantly increase email open rates.
  • Make them Curious: Subject lines that incite curiosity make for high email open rates. The easiest way to do this is to ask a question. The purpose of asking a question is to make the reader want to know more, either because they’re interested, or because they are afraid of missing out on an opportunity.

#2) Split Testing a Subject Line

Resist the temptation to split test multiple variations of a subject line simultaneously. Instead, choose and test a single feature at a time. This will offer you greater accuracy in the result.

Here are some of the most commonly tested features, along with examples of how to test them:

Design: The design of a subject line involves several variables that impact how a subject line looks in the readers’ inbox. The simplest design change, for example, capitalization, punctuation, length, or word order, can significantly impact email open rates.

Here are some ideas for split testing subject lines based on design features:

  • (Capitalization)
    • This dress was made for YOU.
    • this dress was made for you.
  • (Length)
    • Want that perfect tan?
    • If you want that perfect tan, book an appointment at our salon today!

Tone: A subject line’s tone can be altered by changing the voice from active to passive or by using words that imply authority or familiarity to the reader. Infusing humor or a sense of urgency into subject lines, or using literary devices like rhymes and alliteration, can also help to increase email open rates.

Here are some ideas for split testing subject lines based on tone:

  • (Voice)
    • This dress was made for YOU!
    • We made this dress for YOU!
  • (Urgency)
    • Last chance to book your copy of “Book Title”!
    • Book your copy of “Book Title”.

Style: Test subject lines with different styles to find which works best. Some of the most effective subject lines are in the form of questions, news, lists, and statistics. Choosing words that are suggestive of novelty or trends increase email open rates.

Here are some ideas for split testing subject lines based on style:

  • (Form)
    • Book your tickets to Trevor Noah’s concert today!
    • Have you booked your tickets to Trevor Noah’s concert yet?
  • (Lists)
    • Learn how to reduce your carbon footprint.
    • 20 ways you can cut your carbon footprint.
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List Of Top 10 Email Subject Line Testers

Here is the list of popular Subject Line Tester tools:

  1. Mizy by Automizy
  2. Send Check It
  3. CoSchedule
  4. SubjectLine
  5. Advanced Marketing Institute’s Headline Analyzer
  6. ISnotSpam
  7. Email Subject Line Grader by Net Atlantic
  8. Omnisend
  9. Test Subject by Zurb
  10. Touchstone

Comparison Of Best Subject Line Tester Tools

ToolBest ForTop FeatureCostRatings
Mizy by Automizy

Automizy logo
Checking overall copy impact, subject line generationRecommends subject lines based on email copyFree5/5
Send Check It

Send Check It logo
Comprehensive subject line analysis, and optimizing the appearance of subject linesShows a preview of how the subject line will look in different inboxesFree4.5/5
AMI’s Headline Analyzer

AMI logo
Checking for emotional marketing value wordsEvaluates subject lines based on its use of Emotional Marketing Value wordsFree4/5

ISnotSpam logo
Checking for spam triggersConducts SPF, Sender-ID, DomainKeys, DKIM and SpamAssassin checksFree4/5

Review of the tools:

#1) Mizy by Automizy

Best for checking overall copy impact and subject line generation.

Mizy by Automizy

Mizy is a free email subject line testing tool offered by Automizy. This tool uses deep learning and artificial intelligence to scrutinize millions of email marketing campaigns to help you create the best subject line. As a bonus, Mizy not only evaluates the effectiveness of subject lines but also has a subject line generator that can analyze your email copy and suggests a suitable subject line for it!


  • Uses AI and Deep Learning to analyze email subject lines.
  • Recommends subject lines based on email copy.
  • Offers subject line templates to improve email open rates.

Verdict: Mizy’s best feature is that it allows you to feed its generator the contents of your email, then analyze this copy, and create the perfect subject line for your email. The tool also grades your subject line and shares subject line templates to help improve it. This tool is a clear winner.

Price: Free

Website: Mizy by Automizy

#2) Send Check It

Best for comprehensive subject line analysis.

Send Check It

Send Check It is one of the most comprehensive email subject line tester tools available online. Along with an overall score, it offers feedback on sixteen different parameters, some of which are scannability, reading grade level, sentiment analysis, and general ‘spammy-ness’. For every criterion assessed, the tool also offers suggestions for improving your subject line.


  • Analyzes subject lines based on 16 criteria.
  • Scans for potential spam words/phrases.
  • Shows a preview of how the subject line will look in different inboxes.

Verdict: Send Check It is a popular tool on account of its capacity to conduct a thorough analysis of subject lines against multiple measures simultaneously. This tool offers useful tips for the improvement of specific aspects of the subject line, making it a must-have for any marketer.

Price: Free

Website: Send Check It

#3) CoSchedule’s Email Subject Line Tester

Best for checking words that increase opens, character, word and emoji count, case, and numbers.


CoSchedule’s Email Subject Line tester is another useful tool for any marketing team. The tool analyzes certain features of the subject line, such as case, character and word count, numbers, and emojis. Then, along with a score, it offers insights into the impact of your choice of vocabulary. You can choose from their list of ‘positive’ and ‘negative’ words to improve your subject line.


  • Offers word banks for further optimizing subject lines.
  • Allows you to compare past subject lines.
  • Shows a preview of how the subject line will look in different inboxes.

Verdict: CoSchedule is an excellent tool, and its best features are its words banks. The lists of positive and negative words that increase or decrease email open rates. This is definitely a handy one for any email marketing campaign.

Price: Free

Website: CoSchedule’s Email Subject Line Tester

#4) SubjectLine

Best for checking overall copy impact.


Subject Line is one of the most straightforward tools available for testing email subject lines, both in terms of use and feedback. All you have to do is enter your subject line, then fill in some details about yourself and your company, and voila, you’re presented with a score out of 100 for your subject line, along with some pointers on what works and what needs improvement. Best of all, it’s also free!


  • Uses over 800 unique rules to evaluate subject lines.
  • Assesses filtering and deliverability issues.
  • Offers critical feedback and advice.

Verdict: SubjectLine is free as well as easy to use, and not only does it give immediate feedback, it gives relevant comments to help improve copy. No wonder then that it is the top-rated email subject line tester tool according to marketing professionals.

Price: Free

Website: SubjectLine

#5) Advanced Marketing Institute’s Headline Analyzer

Best for checking for emotional marketing value words.

AMI's Headline Analyzer

The Headline Analyzer created by the Advanced Marketing Institute is a one-of-a-kind tool that evaluates copy based on something they call ‘emotional marketing value’. This tool was designed to help marketing professionals create subject lines that appeal to people’s sense of reason, empathy, or spirituality, towards boosting email open rates.


  • Gives email subject lines a score based on its use of Emotional Marketing Value words.
  • Assesses whether the subject line impacts the customer’s intellectual or emotional side.

Verdict: The Advanced Marketing Institute’s Headline Analyzer is an excellent tool that examines email subject lines through a unique lens, allowing you to tweak your copy to provoke a range of responses in your readers.

Price: Free

Website: Advanced Marketing Institute’s Headline Analyzer

#6) ISnotSpam

Best for checking for spam triggers.


ISnotSpam conducts the following checks–SPF, Sender-ID, DomainKeys, DKIM, and SpamAssassin, to ensure that your email doesn’t get sent straight to the recipient’s spam folder. Write your email, choose a subject line, and send it to the email id displayed on their page. Then sit back and wait for their report, available within 3 minutes, on how well your email does against spam filters.


  • Conducts SPF, Sender-ID, DomainKeys, DKIM, and SpamAssassin checks.
  • Analyze both your email and the subject line.

Verdict: There is no point in all the tests you put your subject line though if it still ends up in the recipient’s junk folder simply because it triggered the spam filters. The IsnotSpam tool is, therefore, essential for any marketer.

Price: Free

Website: ISnotSpam

#7) Email Subject Line Grader by Net Atlantic

Best for checking subject line type, character and word count, word mixture and balance.

Email Subject Line Grader

Email Subject Line Grader by Net Atlantic is another unpretentious tool that measures the effectiveness of subject lines against three simple, yet relevant, criteria: a) Subject line type i.e. whether it is a statement, question, list, how-to, etc., b) character and word count, and c) word mixture and balance, for example, action, power, emotion, etc.


  • Grades email subject lines against three specific parameters.
  • Offer tips on how to strengthen subject lines.

Verdict: Net Atlantic’s Email Subject Line Grader is free and very easy-to-use. It offers insights that will help improve your email subject lines without beating about the bush, making it a good tool for small-scale email marketing campaigns.

Price: Free

Website: Email Subject Line Grader by Net Atlantic

#8) Omnisend

Best for checking length, wording, spam alert, scannability.


Omnisend grades the effectiveness of subject lines based on criteria such as length, wording, spam alert, and scannability. It gives your subject line and overall performance score, along with suggestions on how you could improve your subject lines for increased email open rates. It also lets you see how your email subject line will look on different devices.


  • Gives an overall performance score for email subject lines.
  • Analyzes email subject lines based on length, wording, spam alert, and scannability.
  • Shows a preview of how the subject line will look in different inboxes.

Verdict: Omnisend may be one of the simplest email subject line graders, but it helps you find a subject line of the perfect length, and lets you see a preview of it–two mighty features, which make it a powerful marketing instrument.

Price: Free

Website: Omnisend

#9) TestSubject By Zurb

Best for optimizing subject lines for mobiles.

Test Subject by Zurb

TestSubject is a useful tool if you keep in mind that most emails are read on phones now, and people decide whether to open an email based on the sender’s name and subject line. This tool shows you how this information will look in their inbox.

All you have to do is enter the sender’s name, subject line, and some copy, and the tool shows you how your email will appear on different mobile devices.


  • Shows a preview of how the sender’s name and subject line will look on different devices.
  • Currently include iPhone 6, iPhone 6 Plus, and Samsung Galaxy Note 5.

Verdict: TestSubject may be a very basic tool, but is still useful when you think about how many people today consume email content on their phones. The appearance of your message on a smartphone could make all the difference to your overall interaction with potential customers.

Price: Free

Website: TestSubject by Zurb

#10) Touchstone

Best for comprehensive subject line analysis, segmentation, full reporting dashboard.


Touchstone is a very advanced tool that, unfortunately, doesn’t come free. Even to just try it out, you have to buy a package–either single-day, which offers testing for an unlimited list size or a monthly package for list sizes up to 250,000.

This deluxe tool will let you split test subject lines, segment your contacts database, and use a specialized algorithm to evaluate your subject line copy for the highest email open and conversion rates.


  • Creates a virtual simulation of your email database.
  • Analyzes up to 10 subject lines per search.
  • Allows segmentation, and filters by sector.
  • Offers a full reporting dashboard.

Verdict: Touchstone may not be free, but, judging by reviews, it is still absolutely worth it. Depending on the package you avail of, this tool offers benefits you won’t get with any of the other free tools. Touchstone has proven to be successful in predicting near-accurate conversion rates, making it the top pick of marketing agencies.

Price: $19.99/day (Unlimited); $499/month (List size up 250,000)

Website: Touchstone


We’ve looked at some best practices for devising strong subject lines and reviewed the top ten subject line tester tools. Choosing the tool that best fits your need will depend on the size and homogeneity of your contact list and your budget.

Mizy by Automizy is the perfect tool for individuals and small marketing teams, as it not only offers a comprehensive evaluation of the subject line but also recommends a suitable subject line based on an analysis of your subject line, and best of all–it’s free.

Having said that, we found the tools vary somewhat in their evaluation of subject lines, and so it is our recommendation that you use TWO tools to test your subject lines for the most accurate result.

Touchstone is best for larger marketing teams, or agencies with bigger budgets, as it offers the full package, going well beyond simple subject line testing to creating a virtual simulation of your subscriber list, and using proprietary algorithms to predict subject line effectiveness.

Research process:

  • Time is taken to research this article: We spent 15 hours researching and writing this article so you can get a useful summarized list of tools with a comparison of the top 5 for your quick review.
  • Total tools researched online: 20
  • Top tools shortlisted for review: 10
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