How to test plain-text fallback?
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Your HTML email needs a plain-text version as backup. Some email clients won't render HTML, and some users prefer text-only. To test it, you need to see what your plain-text version actually looks like in the wild.
Most email clients let you switch to plain-text mode. Check your email app's settings. In Gmail, look for "Display email as text" in message options. Outlook and Apple Mail have similar toggles. Once enabled, open one of your sent emails. If the text reads naturally without HTML artifacts, you're good. If you see raw HTML tags or mangled links, you've got a problem.
For deeper inspection, check the raw MIME source. Every email has a behind-the-scenes structure with both HTML and text parts. Your ESP or email client usually has a way to view this. Look for the text/plain section and read it line by line. Does it make sense? Are links formatted clearly? Is important content missing?
And Here's the key issue. Most ESPs auto-generate plain text from your HTML. This almost always produces garbage: navigation becomes random lists, buttons turn into bare URLs, and formatting disappears. The fix is to hand-write your plain-text version. Treat it as a standalone email, not a stripped-down version. Write it to flow logically without any visual design helping readers understand it.
What to include. Full URLs where links belong (not just "click here"), strategic line breaks for readability, and clear section breaks using dashes or blank lines. Test on a few popular clients. Plain-text fallback isn't just a legal checkbox. It's a real email experience that real people use.
Related: pre-send testing, dark mode testing, A/B testing.
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