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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

Section titled “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

Section titled “Base URL”

Requests go to the same subdomain you sign in with — just swap api in for app:

  • If you sign in at https://{yourPrefix}.app.tryletterhead.com, your API base URL is https://{yourPrefix}.api.tryletterhead.com. For example, acme.app.tryletterhead.com becomes acme.api.tryletterhead.com.
  • If you sign in at the plain https://app.tryletterhead.com (no subdomain), your API base URL is the plain https://api.tryletterhead.com.

What you can build

Section titled “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, including in bulk for many articles in one request
  • 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

Section titled “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

Section titled “Next steps”

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