top of page

10 Subject Line Tester Tools That Can Help Boost Your Email Open Rate

Updated: Nov 26, 2023

Savvy digital marketers know all too well that an eye-catching subject line doesn't just grab the reader's attention – it also nudges them to interact with the content you're dishing out.

email subject line testers

In a nutshell?

The success of email marketing leans heavily on those snappy email subjects for winning over potential customers and inspiring them to open that email.

Why is that so important? Because even an unbeatable offer doesn't matter much if no one opens your email in the first place.

And (surprise, surprise) getting that click in the first place is actually much harder than it sounds.

According to Walsworth, 33% of people make a quick decision on whether to open an email based on the subject line alone, and 69% will report an email as spam based solely on the subject line.

Given these figures, it’s probably no surprise that Mailchimp estimates that the average email open rate for all industries is just 21.33%.

The impact of a good email subject line on open rates

If the statistics above weren’t enough to convince you of the importance of a good email subject line, consider this:

Email subject lines that are too generic don’t get clicked on

A study conducted by Experian shows that companies using emails with personalized subject lines see a 37 percent increase in unique open rates when compared to emails with non-personalized subject lines.

This personalization can be as simple as Including the recipient's name or other relevant information that can improve engagement.

Lines that are too long don’t get clicked on mobile phones

According to review data published in 99firms, 85 % of recipients will open an email on a mobile device first. The numbers go up to 90% for those aged 25-34.

Why is this important?

Because it means any subject line you write should be optimized for mobile devices, including keeping the character length within an ideal range of 30 to 50 characters.

Subject lines that don’t convey a sense of urgency often get ignored

Research by Invesp found that “subject lines that create a sense of urgency or exclusivity can lead to a 22% higher open rate.” Phrases like "limited time offer" or exclusivity (e.g., invitation-only events) can encourage recipients to open the email promptly.

These statistics demonstrate the crucial role that subject lines play in determining email open rates, spam marking, and overall engagement.

What does an email subject line tester program do? What does it check?

An email subject line tester program is a tool designed to evaluate the effectiveness of cold email subject lines in terms of their ability to engage recipients, drive open rates, and optimize overall email campaign performance.

All those things that cause somebody not to click on an email?

Email subject line testers can identify them and help you change them so your open rate improves.

The most basic ones are length and clarity. A tester will check the subject line length and provide recommendations if you’re not between the ideal 30-50 character length.

It will also check if the subject line is clear, concise, and accurately reflects the content of the email. It may also suggest ways to make the subject line more relevant and attention-grabbing.

Three other big things an email subject tester can analyze include:

1. Use of keywords

Certain keywords can trigger higher open rates.

The tester may analyze the use of keywords in the subject line and suggest adding or replacing them to boost engagement.

2. Emojis and special characters

In certain industries, the use of emojis or special characters can sometimes help a subject line stand out or express emotions better.

But this only works if done carefully.

Mailchimp recommends not using more than one emoji at a time and to “use emojis to supplement words rather than replace them, to make sure your main message gets across.”

3. Spam marking

In addition to their impact on open rates, email subjects also play a role in email deliverability.

Email providers use algorithms to filter out spam, so emails with misleading or spammy subject lines will likely end up in the recipient's spam folder.

Business News Daily points out that it’s not only ridiculous words that get your emails marked as spam — sometimes everyday phrases have the same effect. Some phrases in the list include simple word combinations like "Make $$$," "Free," "Cash bonus", "No money down", "Great offer", and "Order now".

Email subject line testers will catch those spam trigger words and provide you with better alternatives.

For example, instead of a long spammy line (Huge Sale! Shop Now for Discounted Items, Limited Time Offers, and Special Promotions on Selected Merchandise!), the email might suggest a more concise and relevant one (20% Off Spring Sale - Ends Sunday!).

Finally, some email subject line tester programs allow for A/B testing, enabling you to compare different subject line variations and determine which one performs best.

They also save time and effort by providing instant feedback and suggestions, rather than having to manually analyze and refine your subject lines through trial and error.

Tip: If you already prepared your email copy and subject line, you can test its spam score and inbox placement in our free email spam checker tool.

Top 10 Email Subject Line Tester Tools

When choosing an email subject tester, look at the tools offered to choose the one that best fits your needs.

Some tools are simple and basic, checking just for length.

Some offer more advanced features, which might be important if your subject is tricky (how do you tell your customers that you have a sale in place without using spammy money or salesy words?).

Here are our 10 favorites:

An in-depth email subject tester tool with lots of bells and whistles – but still free!

Send Check It compares your subject line with emails sent by marketers to tell you what works and doesn't.

Once you have an overall score for your subject line, it then provides a “scalability” (a measure of how readable your subject line is).

Cool Features:

  1. Tells you the reading grade level of the subject line (you want simple and straightforward, not fancy and hard to read)

  2. Checks for ideal length

  3. Analyzes “sentiment” (if your subject line is positive, negative or neutral)

  4. Checks for spammy words

Price: Free

Send Check It - Email Subject Line Tester

CoSchedule will score your overall subject line quality and rate its ability to result in open and click-through rates.

The tool uses a variety of graphics to rate your subject line and then suggests ways to improve it if the rate is poor.

Cool Features:

  • It helps you “harness the power of emojis”

  • Checks for words that increase and decrease opens

  • Shows how your subject line will look on computers and phones

Price: Free (but you must create an account to access all features)

CoSchedule - Email Subject Line Tester

A basic checker that’s perfect if you just want to check things like word count and word balance.

Cool Features:

  • Tells you whether your subject line sounds too generic

  • Suggests different formats to help your email subjects stand out

  • Checks for word mixture and balance to determine the sentimental value of your email subject line

Price: Free

Net Atlantic - Email Subject Line Tester

Puts emphasis on “emotional marketing value,” analyzing how to make your emails appeal to people’s emotions (whether you’re looking for a surprised, excited, or shocked reaction).

Cool Features:

  • Tells you whether your subject line’s message is intellectual, empathetic or spiritual

  • Explains what the tone of your subject line is best for (sales copy, impact content, and more)

Price: Free

Advanced Marketing Institute - Email Subject Line Tester

Worried about sounding spammy above anything else? Then this is the checker for you.

Cool Features:

  • Checks not only the subject line but also the email itself

  • Conducts several spam checks, including SPF, SpamAssassin Checks, DomainKeys, DKIM, and Sender-ID checks

Price: Free (but requires emailing what you need to be tested)

ISnotSPAM - Email Subject Line Tester

Great if you want your subject line analyzed from 0 to 100% to see where it currently stands.

Omnisend assesses readability, language complexity, and engagement and then suggests improvements to make the subject line more concise and reader-friendly.

Cool Features:

  • Examines the emotional impact of the subject line (and then offers suggestions)

  • Checks for the appropriate use of emojis

  • Bonus feature: Omnisend is one of the few email testers to offer keyword analysis, checking for the presence of relevant keywords that improve open rates

Price: Free

Omnisend - Email Subject Line Tester

Best for teams, larger companies, and advanced testing. Touchstone will test for subject line effectiveness and compare different versions of your subject.

Cool Features:

  • Virtual segment testing

  • Can help predict impressions, clicks, and conversions

  • Will show you how to pass spam filters

  • Allows you to compare your results against a database of over 20 billion other emails

Price: Three tiers with prices going from $69 to $297

Mailshake is an email copy analyzer that looks not only at your subject lines but also the body of your email.

Cool Features:

  • Checks your copy for readability and spammy words

  • Provides ideas for deliverability

  • Provides recommendations on how to improve readability

  • Offers suggestions to avoid spam filters

Price: Free

Mailshake - Email Subject Line Tester

First email subject tester taking advantage of GPT-3 AI technology to help you polish your subject lines.

Cool Features:

  • Scores your email right away

  • Helps you clean up spammy words

  • Provides alternate AI suggestions to any email subject line you enter

Price: Free

Mailmeteor - Email Subject Line Tester

Focuses on avoiding spam filters, but paid features mean it also tracks over time and it’s great for teams.

Cool Features:

  • Grades a number of elements from your email, not just the subject line

  • Tracks in real-time whether your email ends up in the inbox or spam folder

  • Receive itemized feedback about possible content issues

  • Examines your email for risky content that can trigger spam filters

Price: $39/month

GLOCK APPS - Email Subject Line Tester

Frequently Asked Questions

1. What is an email subject line tester?

2. How to use an email subject line tester?

3. Why should I use an email subject line tester?


Start landing in more inboxes. 
Use Warmup Inbox to improve your sender reputation and increase your deliverability.

No credit card needed!

bottom of page