What Email on Acid was
Email on Acid was one of the two tools email people reached for before sending a campaign — submit an email, see how it renders in the clients that matter, fix what breaks, send with confidence. It was the number-two platform in the category behind Litmus, and for roughly $74–99 a month, testing was unlimited. That last part is the part that mattered: you could test, change one table cell, and test again without thinking about it.
What the acquisition changed
Email on Acid was acquired by Sinch and folded into Mailgun Inspect. Existing subscribers migrate from June 2026. The product survived; the pricing model did not.
| Plan | Price | Included | Overage |
|---|---|---|---|
| Inspect Basic | $99/mo | 1,000 previews | $0.15/preview |
| Inspect Premium | $199/mo | 2,000 previews | $0.15/preview |
The arithmetic
A preview is one screenshot of one client, in one mode. So a single honest test across fifty clients in both light and dark is about a hundred previews. On Basic, that is ten test runs a month before the meter starts — and ten runs is not ten campaigns. It is one campaign that needed nine fixes.
That is the part that changes how you work. When each re-test has a price, you stop re-testing. You start guessing, or you batch your fixes and check once at the end, which is exactly when a broken Outlook table is most expensive to find.
“The important work is not the first test. It's the tests after every fix.”
Why we started building
We wanted a flat-price, emulated-first preview loop that a team can run as often as the work needs — and we wanted that same loop available over REST and MCP, so an agent writing an email can test its own output instead of treating email rendering as a blind spot. No incumbent offers an MCP server. That is the second reason this exists.
One thing we want to be clear about
This is not a complaint about either team. Metered pricing is a legitimate way to run a rendering farm — real clients on real devices cost real money per screenshot, and charging per preview is an honest reflection of that cost. Sinch and Mailgun are not doing anything untoward, and Inspect is a capable product.
It just stopped being the tool we needed. So we built the one we did, with a different engineering trade-off underneath it — emulation first, which is cheap enough that unlimited is a promise we can actually keep.
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