Litmus

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Do not build this. It is a device lab, and the clients you most need are the ones you cannot install.

Outlook for Windows renders with the Word engine, and the version matters. Gmail behaves differently in a browser, in its own application, and through a forwarded address. Apple Mail differs by operating system version. Reproducing that means licences, machines and versions nobody can legally assemble.

WHEN TO KEEP PAYING
- A large list, where a broken template is a mistake you cannot take back
- A team where somebody who does not write HTML edits the emails

WHAT YOU CAN BUILD, AND SHOULD — IT COVERS MORE THAN YOU EXPECT

**A real test send, which is free and catches most of it:**
- Addresses at the main providers, and one Outlook on Windows if anybody in the company has one
- Send every template there before it goes out, every time. That single habit finds the majority of what a preview service would have told you
- **The interface refuses to send a campaign that has not had a test send.** That is a rule in code, not a note in a document

**A checklist that runs automatically on the template:**
- No flexbox, no grid, no background image, no external stylesheet, no web font — the Word engine ignores all of them
- Every image has alt text and explicit width and height, because images are blocked by default for a large share of readers
- A plain-text alternative exists and is generated from the same source rather than written twice
- Total size under the threshold where clients clip the message
- Subject and preheader within the lengths that actually get truncated
- Every link resolves, and none of them is the unsubscribe rewritten by accident
- That is a script over the rendered HTML, it runs in CI, and it is the part that scales

**Use MJML** rather than hand-written table markup. It exists precisely because this problem is miserable, it compiles to what works, and it removes a whole class of the errors a preview service would find.

**And check with images off**, which is a browser setting and is how a meaningful share of readers will see it. A message whose only content is an image says nothing to them.

THE ONE-LINE VERSION
Buy the lab if the list is large. Otherwise: MJML, a real test send that is enforced rather than suggested, and a checklist in CI — which is most of the value for none of the cost.