The API, for things that write email without looking at it

Start with MCP, not with HTTP

Most of the traffic here is an agent, so the shortest path is the MCP server . Your assistant gets tools it can call on its own: analyse this HTML, score it across 21 clients, tell me what breaks in Outlook, fix it. It works in Claude, Cursor, and anything else that speaks the protocol, and it compiles React Email, MJML and Maizzle before it checks, so it reads what your recipients receive rather than the source you wrote.

The REST endpoint underneath is the same engine, and it is the right choice for a build step or a service. Both are documented at docs.emailens.dev/api .

Getting a key

API keys come with the Dev plan and above, and you create them yourself in the dashboard . Dev is one key and 100 checks a day; Pro is ten keys and no daily limit. Rate limits are per key.

The web checker stays free and needs no key at all, so you can see what the output looks like before deciding whether to wire it into anything.

Putting it inside your own product

If you are building an email tool and want these checks in it rather than building the data layer yourself, that is a conversation rather than a checkout. The engine is open source and you are welcome to run it. Hosted, at volume, with support and without our name on it, is a licence: contact@emailens.dev .

What it will not tell you

It resolves your HTML against per-client support data. It does not open your email in a real copy of Outlook and photograph it. That is what makes it fast enough to call on every save, and it is also the limit: for a pixel-accurate record of what a specific client drew on a specific day, you want real-device screenshots, a slower, separate thing. They are on the + Real plans.