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Introduction

Local Letter

Transactional email that speaks every language. Design a template once in a visual editor, translate it per locale, and render it from any codebase with one typed SDK call. Open source and self-hosted — your templates, your database, your infrastructure.

await letters.send({
  template: 'welcome-email',
  to: 'customer@example.com',
  variables: { first_name: 'Sagar' },
  locale: 'fr',
})

What it is

Local Letter splits an email into the two things that change at different speeds:

  • The template — subject line and HTML body, edited visually in the dashboard, versioned per locale. Marketing and support can change copy without a deploy.
  • The call site — a single send() in your backend that names a template key, hands over some variables, and says which locale the recipient reads.

Rendering happens in your API. Delivery happens through Resend , called directly from your server, so your Resend key never reaches Local Letter.

The pieces

PieceWhat it does
API (apps/api)Express + Prisma + Postgres. Auth, projects, templates, locales, API keys, and the render endpoint the SDKs call.
Dashboard (apps/web)React + Vite. The visual template editor, translations, projects, and API-key management.
SDKsNode, Python and Go — the same send() in each.
Marketing site (apps/site)The public site at the project’s front door.
Docs (apps/docs)This site.

Where to start

Install and run it locallySend your first emailHow templates and locales fit togetherThe render endpoint

Design decisions worth knowing up front

  • Your Resend key stays yours. The SDK renders against your API, then calls Resend itself. Local Letter never sees the key and never sends mail.
  • A missing variable is loud. An unmatched {{token}} is left in the output rather than blanked, so a mistake is visible in the email instead of silently producing a gap. See Variables.
  • Locales fall back, they don’t fail. Requested locale, then your fallback, then the template’s default. See Locales.
  • Templates are per project. A template key is unique within a project, and an API key is scoped to exactly one project. See Projects.
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Installation