Why email testing is so important – and eight things you need to test

Looking for a fail-proof email test service? Learn more about the essential email testing methods and what specialized services to best use for ultimate reliability.

According to independent estimates, more than 375 billion emails will be sent worldwide each day in 2025. But how many of those will reach their addressees without timely email testing? We can safely say that not a lot.

Yet email remains one of the most common ways of marketing and operational communication with customers and partners with a valid email address.

We are here to show you how you can employ email testing to mitigate failed email delivery risk regardless of your email service provider (ESP). In particular, based on the functionality of email test services like email test service Folderly.com and other platforms focused on maximizing email performance.

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How many people actually view your emails?

Making sure that your marketing letter reaches the addressee before sending it across the mailing list has gradually become a new norm.

Thankfully, we are getting specialized tools that help us see whether our emails hit the spam folder and are viewed by recipients the way we intended. But what indicators to test exactly when it comes to email testing?

Eight things you should test

First of all, make sure that your mailing list consists of real people that can be reached via email in general. For this, you can run a series of manual tests to see how your newsletter works across email platforms, while specialized testing tools should automatically suggest certain common types of tests.

Here are eight things you need to test.

1) Check your email deliverability  



This check is easily carried out via online services, which in most cases have a similar algorithm:

  • Send an email to a special address;
  • Follow the link in the response message, something like this: “View full delivery report.”

To generate a comprehensive report, the email deliverability test usually includes an analysis of headers, reputation of the blacklisted outgoing IP address, email engagement metrics, and SPF (Sender Policy Framework) records.

The process consists of:

  • Checking authentication methods to make sure your outgoing mail server is configured correctly;
  • Analysis of email content using popular spam filters based on included words, phrases, links, images, and code;
  • Source list sampling ─ to take a small sample of real email addresses and use them to check email deliverability through various email services.

2) Check the rendering of the email body

You will need to ensure that communications meet common standards in order to get results. However, the same letter may look different in different email systems. There are multiple reasons for this:

  • operating system and browser updates, which are sometimes unannounced;
  • frequent changes in service providers;
  • shifting design trends to attract different audiences.

In all these cases, it is a must to check the rendering quality of texts and .html content when it is processed by different systems. An email test service evaluates the recipient’s user interface, regardless of where or how they view emails. So rendering and testing go hand in hand when it comes to email marketing ROI.

3) Check what your email looks like on mobile

It’s no secret that the appearance of emails in web clients sometimes differs significantly from their presentation in mobile applications. However, mobile devices are the primary mediums for accessing any information online nowadays.

A good email testing service should provide you with tools that help preview what the recipient of your letters sees on a smartphone or tablet once they get your email. 

This allows you to tailor your email newsletter to the preferences of your target audience and timely seize personalized opportunities to expand your business.

4) Check the rendering of images

Shabby display of email graphics significantly reduces the quality of the information you are trying to convey to a potential buyer or investor. You need to know exactly whether emails with images are filtered, blocked, or redirected to the spam folder by the recipient’s email client.

An email test service compares visualizations with references and employs unit testing to speed up the process. 

5) Check your subject lines

The storylines you deliver can make or break your email campaigns. Subject lines should be concise and accurately convey the essence of your message. Otherwise, emails often end up in the wrong place. The speed of opening emails also depends on their quality, especially if the tastes and behavior patterns of your target audience differ.

You need to spam check an email newsletter, which includes checking case, length, word order, and topic urgency. That being said, it is important to test the same subject line with different categories of potential readers.

6) Check your email segmentation

When organizing a marketing campaign, you want your emails to reach the most relevant demographics. For this, you need to split the mailing list based on addressee similarities in order to send only relevant content and offers. To do this, you need to find a service that provides the necessary tools for high-quality email segmentation.

7) Check the “from” address

Online users are reluctant to open emails from unknown or untrusted senders. And they are absolutely right to do so. When deploying a marketing campaign, you need to make sure that your address does not confuse recipients, and letters go to the inbox. The easiest way to do this is to send multiple emails to specific addresses using different email clients.

8) Check the date and time of sending

It is extremely important to choose the right time to send emails. It happens that your letter lies unread for half a day, and your proposal may lose its relevance. To avoid this, you need to segment your list of recipients based on age, peak online activity time, and so on.

By tracking the click-through rate of emails sent over time, you can determine when users are most likely to view them and most likely to click on embedded links. This information will help you optimize your marketing campaigns, allowing you to schedule your mailings at the most efficient time for your subscriber base.

Popular email testing services

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And now for a few words about the top email testing tools, in our opinion, to carry out all the above checks and tests.


The most common online email testing service up to date, thanks to its functionality, a large selection of rendering tools, a dedicated expert blog, and video content. The provider explores features of over 90 email clients, keeps track of system updates, and market trends. You may try out a trial version for free before paying for the service.

Email on Acid 

A very popular tool packing some of the most commonly requested features, such as:

  • Testing and previewing emails on different clients and devices;
  • Power editor;
  • Spam tester;
  • Blog with expert articles

Email on Acid is a paid tool with a free trial version available.

Mail Tester

Accessible and functional. This one does not display emails on different devices, but highlights errors that cause the mailing list to hit spam folders. The email checker evaluates letters on a special scale. At the same time, the history of sendings and statistics is saved. Thanks to SpamAssassin, Mail Tester is mainly configured to check newsletters, but works well with single emails as well. And it does not require registration and authorization.


A promising email test service Folderly.com with promptly responsive support. The tool can be extensively customized to achieve the simplest, most intuitive user experience. Works with the email infrastructure, processing a huge amount of information: it analyzes the previous activity of the sender and the reputation of the domain, and shows what needs to be changed.

Always test your emails

Automated marketing campaigns are essential to any business, so testing emails before sending them should be a top priority.

Trusted platforms like Forderly analyze the patterns in your emails to find the root cause of poor deliverability. These are your trusty tools to make the addressee’s inbox every time, and you can fix common delivery issues in a couple of simple steps.