Documentation
Everything you need to plug your SaaS into Letter: report users and events from your backend, capture leads with forms, then drive sequences and broadcasts off of them.
Getting started
Agent setup Hand this to your coding agent: install, authenticate, instrument in one pass Read → MCP server Connect Claude, ChatGPT, GitHub Copilot, or any MCP client to https://mcp.letter.app/ and manage sequences, broadcasts, forms, contacts, and settings from chat Read → Initial import Bulk-load existing users and accounts from your SaaS in one shot Read →
Guides
Forms Capture external leads with hosted, embeddable forms: fields, double opt-in, target lists, spam protection, the submit-to-sequence flow, and managing forms from the CLI and Management API Read → Sequences Author drip sequences end to end: triggers, the node graph, branch logic, email bodies, validation, and a full worked example for the CLI and Management API Read → Broadcasts Send a one-off email to a segment: build an audience from traits, events, and lists, preflight the recipient count, schedule or send now, and track delivery live, from the dashboard, CLI, or Management API Read → Email templates Visual, text-only, and custom HTML layouts. Custom HTML must include {{{content}}} for the body slot and should include {{unsubscribe_url}} (or Letter appends a footer). Create them from the dashboard, CLI, Management API, or MCP. Read →
API
Ingestion API identify · group · track · batch · auth · rate limits Read → Transactional email POST /v1/send · receipts, resets, notifications · migrating from Postmark Read → SMTP smtp.letter.app · same project key as POST /v1/send · Twenty, WordPress, Laravel Read → Management API control-plane for projects, contacts, segments, sequences, broadcasts, templates (including custom HTML layouts with {{{content}}} and {{unsubscribe_url}}), and settings Read →
SDKs
SDKs Official server-side clients for Node, Python, and Ruby. Report users and events, and send transactional email, idiomatic in each language, with batching, retries, and idempotency built in. Read → Node SDK Auto-batching client for Node.js with retries and idempotency Read → Python SDK Auto-batching client for Python with retries and idempotency Read → Ruby SDK Auto-batching client for Ruby with retries and idempotency Read →