How do testing tools detect accessibility issues?
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Picture someone on your list who's blind. They open your email with a screen reader, a tool that reads the content aloud. If your images have no alt text, the screen reader announces "image" and moves on. If your link says "click here", they have no idea where it goes. Your beautifully designed email is basically noise.
Accessibility testing tools exist to catch those problems before your email lands in anyone's inbox. Here's how they actually do it.
Parsing the HTML structure
The tool reads your raw HTML the same way an email client would. It checks whether images have alt attributes, whether those attributes are empty (which is intentional for decorative images) or missing entirely (which is a problem). It also looks at heading order, table structure, and whether elements are tagged semantically so assistive technology can navigate them.
Color contrast analysis
Tools calculate the contrast ratio between your text color and background color, then compare that number against WCAG thresholds. Normal body text needs a ratio of at least 4.5:1. Large text gets a little more flexibility at 3:1. If your light grey copy sits on a white background, the tool flags it. This matters for people with low vision or color blindness, which affects around 8% of men and 0.5% of women globally.
Link and button text evaluation
But the tool scans every link and button in your email for meaningful label text. "Click here" or "read more" fails because it tells a screen reader user nothing about the destination. The tool flags these and asks you to use descriptive text instead, like "Read our winter shipping guide".
Reading order logic
Still many email layouts use tables for structure. Visually that looks fine. But screen readers follow the DOM order of the HTML, not the visual layout. A tool can flag situations where the reading order in the code doesn't match the visual reading order, which creates a confusing experience for screen reader users.
What tools can't do automatically
This is the honest part. Automated tools catch technical violations, but they can't tell you whether your alt text is actually helpful. An image of a product with alt text that says "img_23847.jpg" technically passes the "alt attribute exists" check. It fails every real-world test. The same goes for whether your content still makes sense when read linearly without visual context.
That's why automated testing is a floor, not a ceiling. Run the tool, fix the flags, then test your email with an actual screen reader like VoiceOver on Mac or NVDA on Windows. That's how you find the gaps that code scanning misses.
You can also run your HTML through our free Accessibility Checker to get a quick read on what your email is missing before you hit send.
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