Should You Take Help From Subject Line Testers?

When it comes to emails, it is literally a matter of ‘Subject’!

Email marketing has a lot of defying aspects to what we call a successful campaign. From email template design elements to an appealing copy, deliverability, engagement metrics, and so on. During all these years of email marketing, and that is more than 50 years – it has always evolved with consumer behavior and market trends. Out of all the things that stayed constant, a compelling ‘Subject Line’ never got out of the picture! An email’s open rate is dependent on its subject line, and all the other metrics follow. When we are struggling with a very short attention span and cluttered inboxes, an eye-catchy subject line is a MUST. 

To validate how vital it is to put in efforts while crafting an engaging, personalized, and relevant email subject line, have a look at these stats below:

  • 47% of recipients open emails solely based on the subject line.
  • Nearly 70% of people report an email as spam based on its subject line alone.
  • Personalized email subject lines have the potential to boost email open rates by 50%.
  • Emails subject line with the first name of the recipient observes a higher click-through rate than those without.

These numbers throw light on how important it is to pay attention while crafting an email subject line. Clearly, it has the power to make or break your email campaign’s success. With technology breakthroughs and advancements, email marketing experts across the industry have come up with ‘Email Subject Line Testing’ tools claiming to mould subject lines to be more performance-driven and accurate. 

Today, we are going to learn about how various Subject Line Testers work, why is it important to test your subject line, the strengths, and the shortcomings. Let’s get to it.

The TEST of Subject Line Testers

To be honest, the competition isn’t any less in here too. There are plenty of paid and non-paid subject line testers available in the market. Moreover, email service providers (ESPs) like Mailchimp offer a built-in subject line tester for the businesses that craft their email template and design a campaign in Mailchimp.


To have a brief idea about how different subject line testers work, we ran a test on 3 of the prominent subject line testers. We tested a single subject line on CoSchedule Email Subject Line Tester, Omnisend Email Subject Line Tester, and Moosend refine tester. Here’s what we got.

Tested Subject Line: Email Uplers Wants To Thank You! 

1. Coschedule Tester

2. Moosend Result

3. Omnisend Result

The Score Overview

While all the three testers gave different results, the parameters on which CoSchedule and Omnisend rate the email subject lines scores seem similar. Wherein Moosend has a different perspective. The scores fell somewhere between 50-80 which shows a drastic change. Moosend on the other hand provides industry open rate comparison which is fair at 4.93% above the industry’s average.


All three testers work upon their own algorithms, best practices, and stored data. When you test subject lines, you need to identify what suits your industry and how much you can count on the results.

Improvement Suggestions by the Testers

1.CoSchedule Suggestions    

2. Omnisend Suggestions

3. Moosend Suggestion

The Suggestion Overview

Both CoSchedule and Ominsend seem to run on similar algorithms and thus provide similar subject line improvement suggestions. Both suggest to add or remove power words & negative words in order to improve the deliverability and open rate. On the other hand, Moosend is more focused on personalizing the subject line by adding the first name and including a curiosity component.


The word and character count suggestions have a positive impact as that should be a focal point while writing subject lines. 82% of the marketers try to keep their subject lines short and concise up to 60 characters or less because users today use mobiles to access their emails, and in compliance with the smaller screen resolutions, subject lines with lesser characters tend to perform better. It is interesting to know that the positive and negative word libraries differ from tester to tester, and for that matter, industry to industry. You cannot restrict your subject line text based solely on these words. And as far as using an emoji or number is concerned, that is considered as subject line best practice. Precisely, Moosend is the only tester that encourages to personalize the subject lines, which is an important email open-rate factor too.

Subject Line Previews


CoSchedule and Omnisend again rank at par with showing desktop and mobile previews. Moodsend, on the other hand, has no such provision.


Previews play a vital role while you craft your subject lines. How it will appear on various devices is an important factor to consider. CoSchedule gives 3 email client variants including Gmail, Outlook, and Mobile, whereas Omnisend shows 2 of them. Keeping the email client interface in mind is imperative in order to predict the impact of your email subject line. The important words should be visible in all the devices across the clients.

Words of Wisdom

We just compared 3 of the testers from the pool, and 2 of them appeared to be almost similar. There are plenty with or without these parameters. What we could derive from undertaking this test is:

  • There is no perfect tester on which you can solely rely. In fact, these testers might drive wonderful results for a fashion brand while the finance or travel industry might struggle to get the same results. You need to identify which tester works best for you. Just like email template designs, A/B testing is THE most reliable source for testing subject lines as well.
  • The data on which these testing tools run are static and not dynamic. They calculate through the fed data and revolve around old findings (if not updated frequently). Email marketing trends change with time, industry, audience, and even ESP features. It is advisable to trust your own ESP subject line testers that can form results based on your past campaign.
  • Email subject line testers play a prime part when it comes to optimizing the text based on the preview and character & word count. Compelling email subject lines are as imperative as responsive HTML email templates.
  • Create your own data against which you can measure the success ratio of your email subject lines.
  • A bonus tip is to take help of AI algorithms to figure out what kind of subject lines can work well for you. (Phrasee is a nice tool to start with.)

Wrap Up

While crafting your email subject lines, following the general best practices is the key! Subject line testers should be used as a secondary helping hand and not a primary tool to rely on. With that being said, we hope this guide on using email subject line testers proves to be helpful for you. Happy mailin’!     

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