Using the Letterhead API

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The Letterhead API lets you work with your newsletters from your own code — your audience, your content, your sending, and your reporting. Everything you can do in the dashboard, you can build around.

Authentication

Every request needs an API key sent as a Bearer token in the Authorization header:

Authorization: Bearer <your-api-key>

One key reaches every newsletter in your account, so you don’t need a separate key per newsletter. See Generate & manage API keys to create one.

Most read endpoints also expect api=true in the query string. It tells Letterhead you’re authenticating with a key rather than a signed-in session, and the reference notes it wherever it applies.

Base URL

All requests go to:

https://api.tryletterhead.com

What you can build

Your audience — add, update, and remove contacts, set their subscription status per newsletter, and tag people in bulk
Segments — create and update saved audience groups, count who’s in one, preview the members, and pull engagement over time
Content — push articles into your curation library and distribute them across newsletters
Sending — draft a letter from your own HTML or from a template, then send it
Promotions — create and manage the paid placements running inside your newsletters
Reporting — pull opens, clicks, bounces, and unsubscribes, with a daily breakdown and per-newsletter detail
Audience overlap — shared subscriber counts for every pair of newsletters, and combined unique reach for a selection

API reference

The complete endpoint reference — paths, parameters, request and response formats, and examples — is in the API reference on this site. It’s generated from the API’s own specification, so it stays in step with what the API actually does.

Next steps

Still can’t find what you need? Contact support.