Email testing

Email testing means checking an email before it goes out: that it looks right in the email clients your readers use, that every link and image works, that it holds up in dark mode, and that nothing will get it clipped or flagged. This guide covers what to check, how to check it, and which tools help.

1Check how it looks in each email client

Email clients don't all display HTML the same way. The same email can look different in Apple Mail, Gmail and Outlook, because each one supports different HTML and CSS.

• Apple Mail (iPhone and Mac) uses WebKit, the engine behind Safari, so it supports most modern HTML and CSS. It's one of the most widely used email apps.
• Gmail removes some CSS, rewrites parts of your code, and clips long emails (see section 4).
• Outlook.com and Yahoo each have their own quirks with spacing, fonts and images.
• Classic Outlook for Windows lays out email with Microsoft Word's engine, which is why emails are still built with tables.

You don't need to test every client in the world. Look at your audience: most email platforms report which clients your readers open in. Test those, and test them every time you change a template.

2Check it in dark mode

A lot of people read email in dark mode, and clients handle it in three ways: some leave your colours alone, some invert them, and some, like Apple Mail, use the dark styles you give them. Check for:

• dark logos that vanish on a dark background
• text that becomes unreadable when only the background changes
• images with white edges that look like boxes

Our guide to Apple Mail dark mode has the code to fix the common problems.

3Check every link and image

Broken links and missing images are the most common mistakes, and the easiest to catch:

• every link returns a page, not a 404 or a timeout
• links use https, not plain http
• images load from a public address, not a local file or a staging server
• each image has alt text, for screen readers and for people with images turned off
• images aren't huge: a 3MB hero image is slow on a phone

4Check the size, so Gmail doesn't clip it

Gmail cuts off any email whose HTML is over about 102KB and hides the rest behind a "View entire message" link. Your unsubscribe link usually goes with it. Your email platform's tracking links add weight after you export, so leave headroom.

Check your email's size for free, or read why Gmail clips emails and how to stay under 102KB.

5Check accessibility

Accessible emails are easier for everyone to read:

• set the language on the <html> tag and give the email a <title>
• use real text, not text inside images
• keep enough contrast between text and background
• write link text that makes sense on its own ("Reserve a loaf", not "Click here")

6Check the details that affect the inbox

• Preheader: the short preview text shown after the subject line. Without one, clients show whatever text comes first.
• Subject line: avoid capitals and strings of exclamation marks, which look like spam.
• Mobile: include a viewport tag so phones don't show the email zoomed out.
• URL shorteners: bit.ly-style links are a strong spam signal. Link to the full address.

7Test what your platform actually sends

Your email platform (Mailchimp, Klaviyo and so on) changes your HTML when it sends: it rewrites links for tracking, adds a tracking pixel and inserts personalisation. So the email people receive isn't quite the file you built. Send a test from the platform itself and check that version too.

How to test: by hand or with a tool

By hand: create free accounts at Gmail, Outlook.com and Yahoo, send the email to each, and check it on your own phone and computer. It works and costs nothing, but it's slow, you only see the devices you own, and it's easy to miss a broken link.

With a tool: an email testing tool shows your email in several clients at once and runs the checks for you. The big tools, Litmus and Mailgun Inspect (formerly Email on Acid), cover 100+ clients and devices and start at around $99 a month. Lighter tools cover fewer clients for much less.

EmailTestr is one of the lighter ones, and we make it: paste your HTML or send it from your email platform, and in a few seconds you see it in Apple Mail on iPhone and Mac, light and dark, with a list of what's broken and the line to fix. It's free during early access. For honest comparisons, see EmailTestr vs Email on Acid, vs Mailgun Inspect and vs Litmus.

A checklist to copy

1. Previewed in the clients your readers use, on phone and desktop
2. Checked in dark mode
3. Every link works and uses https
4. Every image loads and has alt text
5. HTML under 102KB, with headroom for tracking links
6. Language, title and readable contrast set
7. Preheader written, subject line calm
8. Viewport tag in, no URL shorteners
9. A test sent from your email platform, and checked