8 Best Email Subject Line Testers To Use Before You Send Newsletters

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Crafting compelling email subject lines is an art. With emails overflowing people‘s inboxes, grabbing their attention with your subject line is critical.

Research shows 33% of email recipients decide to open an email based solely on the subject line [1]. Another study found 69% of respondents would report an email as spam after only reading the subject line [2].

So taking the time to test and optimize your subject lines is well worth the effort.

In this guide, we‘ll explore 8 excellent tools to analyze and improve your email headlines. These subject line testers evaluate your copy on key metrics like:

  • Emotional word usage
  • Spam trigger terms
  • Headline length
  • Readability

Let‘s dive in and see how these tools can help you write subject lines that convert more opens and clicks!


Omnisend offers an AI-powered subject line analyzer. It scores your headline on a 0-100% scale, along with actionable suggestions.

For example, here‘s how it evaluates the subject "Recipe blog update: revenue up 45% from last year!":

![Omnisend subject line analysis](

Helpful metrics provided include:

  • Headline length breakdown (words, characters)
  • Readability grade level
  • Spam keyword warnings
  • Capitalization checks
  • Number of emojis

Omnisend also shows previews on desktop and mobile, so you can ensure your subject line renders well across devices.


CoSchedule‘s headline analyzer focuses on emotional impact. It scores your subject line based on positive vs. negative wording.

The tool suggests using more uplifting terms from its curated vocabulary bank. It also provides useful data like:

  • Recommended case formatting
  • Character and word count
  • Number of emojis
![CoSchedule subject line scoring](

For boosting open rates, CoSchedule checks for power terms that compel action. It also previews how your headline will appear in the inbox on desktop and mobile.

![CoSchedule preview](

Overall, CoSchedule is ideal for optimizing the emotional impact and readability of your subject lines.

Send Check It

For in-depth subject line analysis, Send Check It is a great option. It compares your headline against a database of 100,000+ marketing emails to benchmark performance.

The tool analyzes your subject line across 16 factors including:

  • Headline length
  • Readability
  • Emotional sentiment
  • Spam words

You get a spam score showing how risky your subject line is, along with ways to improve it. Send Check It also provides mobile and desktop previews of how your headline will appear.

![Send Check It subject line analysis](

For well-rounded feedback to boost open and click rates, Send Check It is an excellent choice.


Mizy is an AI-powered subject line rating tool by Automizy. It analyzes your headline against a database of high-performing email campaigns.

Using deep learning algorithms, Mizy scores your subject line from 0-100 based on predicted performance. It gives actionable suggestions tailored to your headline.

![Mizy subject line rating](

Since it‘s still in beta testing, Mizy has some kinks to work out. But it shows great potential as an AI assistant for writing effective subject lines.


With email reading happening more on mobile, Zurb‘s Test Subject tool is handy. It lets you preview how subject lines will render on smartphone displays.

Test Subject ensures your headlines are mobile-friendly by showing previews on various devices like:

  • iPhone
  • Android phones
  • Apple Watch
![Test Subject mobile headline preview](

This tool focuses specifically on the mobile experience. It‘s ideal for ensuring your emails headlines are formatted properly across mobile devices.


Mailshake‘s headline analyzer offers an easy way to improve email deliverability. It checks your entire email copy for issues like:

  • Spam trigger words
  • Improper link formatting
  • Overuse of CAPS

You get an overall spam rating, along with fixes for optimizing deliverability. Mailshake also makes suggestions to boost open rates, like using personalized subject lines under 30 characters.

![Mailshake subject line and copy analysis](

It‘s a great tool for easily checking subject lines and email copy to dodge spam filters.


As the name suggests, SubjectLine offers a quick subject line analysis. It provides an overall quality score based on 800+ rules.

The tool checks for issues like:

  • Blacklisted terms
  • Improper formatting
  • Excessive length

You get actionable tips for improving deliverability and open rates. SubjectLine won‘t analyze your headline in depth. But it‘s ideal for a high-level subject line check.

![ subject line analysis](


To check your subject line for spamminess, IsNotSpam is handy. It runs analysis on factors like:

  • SPF records
  • Sender ID checks
  • DKIM signatures

You get warnings for any spam triggers found in your headline or email body. IsNotSpam helps you identify any terms, links, or formatting that could get flagged as spam.

Key Takeaways

Here are a few key tips for writing email subject lines that convert:

  • Use emotional words – Target feelings like excitement, urgency, or exclusivity. But avoid coming across as too salesy.

  • Add numbers – Research shows including numbers like timeframes or statistics boosts open rates.

  • Test mobile rendering – Ensure your headlines render well on mobile devices. Over half of emails are opened on mobile.

  • Limit to 30-50 characters – Shorter subject lines often outperform longer ones.

  • Avoid spammy words – Use subject line testers to detect and eliminate spam trigger terms.

  • Personalize – Include the recipient‘s first name or company to boost open rates. Segmentation helps.

  • Use power words sparingly – Power words prompt action, but overusing them can backfire.

  • A/B test subject lines – Try different versions to see which resonates most with your audience.

  • Check the emotional sentiment – Does your headline evoke positive or negative emotions? Tools can detect this.

By researching your audience and testing subject lines, you can find the right fit. Grab readers‘ attention, convey value, and deliver results with compelling email subject lines.


Written by Alexis Kestler

A female web designer and programmer - Now is a 36-year IT professional with over 15 years of experience living in NorCal. I enjoy keeping my feet wet in the world of technology through reading, working, and researching topics that pique my interest.