Man sending an email.

Why every marketing team needs an email testing tool

Marketers today compete for attention in an inbox that now receives over 376 billion emails per day worldwide¹. That figure is projected to rise to more than 408 billion by 2027¹. 

Alongside this, the global email user base has reached 4.6 billion people in 2025², representing well over half of the world’s population. With this level of activity, email marketing campaigns continue to deliver one of the highest ROIs of any digital channel, making rigorous testing more critical than ever.

But to achieve meaningful results, it isn’t enough to just create compelling content. Emails need to display properly on every device, clear spam filters, and protect your sender reputation. That’s why testing is essential. It makes sure every campaign reaches the inbox and performs as intended.

 

The purpose of email testing tools

Person sending an email.

An email testing tool allows marketing teams to preview and stress-test campaigns before hitting send. By simulating how emails render across different devices, operating systems, browsers, and email clients, teams can catch and fix issues that could damage campaign performance.

Beyond design previews, the best email testing tools also work as spam testing tools. They review subject lines, run HTML testing, and check links, images, accessibility, and load times. This creates a safe environment to refine campaigns before launch, improving email deliverability and protecting brand reputation.

The business benefits of email testing 

1. Ensure consistency across email clients and devices

Emails display differently in Outlook, Gmail, Apple Mail, and multiple inboxes on mobile devices. Email testing tools help email marketers preview and optimise designs so that formatting, images, and calls-to-action render consistently everywhere.

2. Avoid spam traps with pre-send checks

With nearly half of all global email volume classified as spam³, it’s essential to avoid content that could trigger junk folder placement. Testing platforms evaluate subject lines, HTML structure, and sender reputation against recognised spam traps and filtering rules. This analysis provides clear guidance on how to improve inbox placement and reduce the risk of emails being diverted into junk folders.

3. Protect sender reputation and brand trust

Customers lose confidence fast when they spot broken links, clumsy formatting, or obvious typos. The stakes are high: almost half of all global email traffic is already marked as spam³, so one careless mistake can push your message into the junk folder. Testing before you send keeps campaigns polished and professional, protecting your reputation with both your audience and the inbox providers that decide where your emails land.

4. Improve engagement and drive ROI

Emails that look right, no matter where they’re opened, stand a far better chance of being read and acted on. Features like subject line previews and link verification support higher open and click-through rates. When combined with segmentation and automation, campaigns that undergo thorough testing have been shown to generate over 300% more revenue⁴ than untargeted sends.

5. Save time and reduce costs

Manual live testing across multiple inboxes is time-consuming and inefficient. Testing platforms cut that down to minutes by showing previews in one place and flagging issues early. They also let teams and stakeholders check campaigns together, which speeds up sign-off and avoids expensive mistakes. The result is less wasted time, smoother workflows, and quicker launches.

 

Recommended email testing tools

Email testing platforms continue to evolve, and these in particular stand out:

Litmus

Litmus is one of the most established names in email testing. It gives you previews across more than 100 clients and devices, plus tools for spam checks, subject line testing, and detailed analytics. It plugs into major marketing automation platforms and everyday collaboration tools like Slack and Google Drive, which makes it especially useful for teams working on large or complex campaigns.

Email on Acid (EOA)

Email on acid boasts unlimited testing, strong collaboration features, and real-time commenting. Excellent for teams running frequent, multi-market campaigns.

Mailtrap

Combines email testing with deliverability monitoring and HTML validation. It’s developer-friendly and well suited for teams sending transactional emails alongside marketing campaigns.

For smaller teams, PreviewMyEmail also offers affordable plans with robust preview capabilities.

Tip: To find out which platform is best for you, make sure you take full advantage of the free trials offered by the different testing platforms. This way you can get hands-on and work out for yourself which platform best suits your needs.

 

Real-world example: Harley-Davidson case study

At h2o creative, we manage Harley-Davidson’s weekly marketing emails across 13 European markets in 6 languages. To ensure flawless execution, every email template is built in Salesforce Marketing Cloud and tested in Email on Acid before send.

The results speak for themselves:

  • 831% ROI increase
  • 154% uplift in email-attributed revenue

This demonstrates the measurable business impact of rigorous email testing at scale. Check out the the full case study here.

 

Best practice checklist for email testing

Before your campaign goes live, check the essentials:

  • Preview across key devices and inboxes
  • Fix broken links and missing images
  • Run a spam filter check
  • Test subject lines for clarity
  • Check load speed and dark-mode rendering
  • Confirm accessibility (alt text, fonts, colour contrast)
  • Get final approval from stakeholders

 

In takeaway 

Email testing is part of doing email marketing properly. It ensures your campaigns land in the inbox, display the way you intended, and deliver the return your team is aiming for. By protecting your reputation, improving engagement, and driving ROI, testing gives marketing teams the confidence that every send counts.

At h2o, we’ve seen first-hand how strategic email testing drives measurable business impact. To explore how we can help elevate your email marketing campaigns, take a look at our services and recent case studies. When you’re ready, get in touch. 

 

Sources

EmailToolTester – How many emails are sent per day (2025)

Virfice – Email marketing statistics 2025

EmailToolTester – Email usage statistics (spam data 2025)

EntrepreneursHQ – Email marketing automation revenue uplift

Frequently asked questions

What are the key features to look for in an email testing tool?

The key features to look for are previews of as many clients and devices as possible, spam testing, link and image validation, as well as good collaboration features. Some other useful features to look for would be load time analysis, an easy-to-use interface, good customer support and integration capabilities.

Is there a limit to how many emails I can test?

This will depend on which tool you use and which plan you are on. On a free plan or free trial, typically there will be a limit on how many emails you can test, whereas most paid plans will be unlimited. For example, Email on Acid’s paid plans offers unlimited testing.

How does spam testing work?

Avoiding spam filters is crucial for email deliverability. A lot of these tools will have spam analysis features and will check for anything that might raise a red flag. They do this by assessing your email content for elements that might trigger spam filters, such as excessive capitalisation, suspicious keywords, or certain formatting issues.

What is an email preview?

Typically, an email preview is a screenshot from a live client, allowing you to see exactly what your subscribers see. Most tools will offer a range of different devices, browsers, operating systems and email clients. For example, both Litmus and EOA offer 90+ different previews.

Is there a free email testing tool available?

Some tools will offer a completely free plan. However, these free plans will come with very reduced features and a maximum amount of test emails that you can run. These are great to see if the platform will be well suited to you or if you have a very limited budget and don't need to do much testing.