Your agent can email.
Now it can mail.
Agent Letter is an API that lets an AI agent send a real, physical letter with one call — printed, address-verified, mailed, and tracked. It's the postal equivalent of giving an agent an email address, built for the documents that are legally required to be on paper.
await agentletter.send({ to, body, certified: true })What it is
What is Agent Letter?
Agent Letter is a physical-mail API for AI agents. It gives an autonomous agent the ability to send a real letter through the postal system with a single API call — the same way services like AgentMail give agents an email inbox. You hand it a recipient and a document; it prints, verifies the address, applies postage, mails it, and streams delivery status back to your agent.
Today, AI agents can draft a document in seconds but cannot put it in the mail. The communications that matter most in the physical economy — a debt notice, a patient's lab result, a KYC document, a legal demand, a government form — are required by law to travel on paper. Agent Letter is the missing piece of infrastructure that lets agents take that action safely and compliantly.
- One verb
- The whole API is send() — call it like an email.
- Built for agents
- Agent identity, budgets, guardrails, and a native MCP tool.
- Compliance-grade
- Address verification, certified mail, and proof of delivery.
Why physical mail
Why would an AI agent need to send paper?
Email solved instant, free, low-stakes communication: receipts, notifications, confirmations, marketing. But a large class of documents cannot legally be an email. They need a postmark, a signature, or court-recognized proof that they were delivered.
As agents move from drafting text to taking real actions, they increasingly reach a step that has always dropped to a human: now put this in the mail. A collections agent generating a notice, a healthcare assistant sending a lab result, an onboarding agent delivering KYC documents — each needs a physical letter, on schedule, with an audit trail. Agent Letter turns that human step into a single API call.
This is new, incremental demand: every letter an agent sends is volume that did not exist before. We don't depend on overall mail growing — we're a new kind of sender layered on top of the world's largest postal market.
How it works
From an API call to a mailbox, in three steps
You call one endpoint. Agent Letter handles printing, address verification, postage, carrier hand-off, and proof of delivery.
- 01
Your agent calls the API
Send a recipient and a document — PDF, HTML, or a template — through the SDK, REST, or a native MCP tool. The shape is identical to sending an email, so it drops into any agent framework.
- 02
We print, verify, and mail it
We validate and correct the address before anything prints, produce the letter on real stock, apply postage, and hand it to the carrier — using compliant facilities for regulated mail like PHI or financial notices.
- 03
You get tracking and proof
Every status change streams back as a webhook. Certified and registered options add signature capture and legal proof of delivery, and return mail comes back to your agent as structured events.
The API
The whole API is one verb: send
If your agent can send an email, it can send a letter. SDKs for TypeScript and Python, a REST endpoint, and a native MCP tool so any framework can call it directly.
import { AgentLetter } from "agentletter"; const agentletter = new AgentLetter(process.env.AGENTLETTER_KEY); // Your agent decides a physical letter is required.const letter = await agentletter.send({ to: recipient, // verified before printing body: pdf, // or { html } / { template } certified: true, // proof of delivery}); letter.id; // "ltr_9f2a"letter.status; // "in_transit"Use cases
Where a letter isn't optional
The communications email can never serve — the ones that are legally required to be physical. That's the demand agents are about to create.
Healthcare & PHI
Lab results, appointment and discharge notices, and patient communications mailed from HIPAA-compliant facilities under a BAA.
Legal notices
Demand letters and service of process with certified delivery and court-recognized proof of service.
KYC & identity
Verification codes and identity documents delivered to a confirmed physical address.
Debt & collections
Regulated collection and billing notices that must be sent on paper, on a compliant schedule.
Government & finance
Regulated disclosures, official statements, and correspondence with a complete audit trail.
Insurance
Policy changes, cancellations, and claims correspondence with delivery confirmation.
How it compares
Agent Letter vs. email vs. a generic mail API
Email can't produce legal paper. Generic mail APIs (Lob, PostGrid) were built for marketers and developers, not agents. Agent Letter is the layer in between.
| Capability | Agent Letter | Mail API | |
|---|---|---|---|
| Legally valid on paper | |||
| One-call API for agents | |||
| Native MCP tool | |||
| Agent identity & delegated authority | |||
| Abuse guardrails & per-agent limits | |||
| Certified mail & proof of delivery | |||
| Two-way loop (returns, signatures) | |||
| HIPAA / SOC 2 compliance |
Why now
Agents just learned to act in the real world
Agentic AI crossed from observing to acting, and the infrastructure that gets built next is whatever lets agents take the actions humans care about. Investors are already funding the communication layer for agents — email first. Physical mail is the highest-stakes channel still unbuilt.
- 27% → 65%
- Share of agent tool-calls that take real-world actions, up in 16 months.
- $6M
- Seed raised by AgentMail (General Catalyst, YC) to give agents email.
- #1
- The US remains the world's largest mail market.
FAQ
Questions, answered.
Give your agent a mailbox
Email was the easy channel. We're building the one that's legally load-bearing. Agent Letter is in private beta — request early access and we'll onboard design partners in waves.