API integration

For developers building AI agents with OpenClaw, n8n, Make, or any agent framework. This guide covers the multi-tenant pattern: create inbox, inject env vars, route inbound by inbox_id. For LangChain or Vercel AI SDK, see Agent frameworks for ready-made tool definitions.

Prerequisites

OpenMail account with API key
An AI agent or agent framework (e.g. OpenClaw, LangChain, n8n)

Use this pre-built prompt to get started faster.

Open in Cursor

Step 1: Create an inbox

One API call. Create an inbox and map inbox_id to your user/container in your app for routing.

const fs = require("fs/promises");

const OPENMAIL_API_KEY = process.env.OPENMAIL_API_KEY;

async function createAgentInbox(userId, containerEnvPath) {
  const response = await fetch("https://api.openmail.sh/v1/inboxes", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${OPENMAIL_API_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ mailboxName: userId }),
  });

  if (!response.ok) throw new Error(`OpenMail: ${response.status}`);

  const inbox = await response.json();
  // inbox.id      → "inb_8f3a1b2c"
  // inbox.address → "jane@openmail.sh"

  await fs.appendFile(
    containerEnvPath,
    [
      `OPENMAIL_API_KEY=${OPENMAIL_API_KEY}`,
      `OPENMAIL_INBOX_ID=${inbox.id}`,
      `OPENMAIL_ADDRESS=${inbox.address}`,
    ].join("\n") + "\n"
  );

  return inbox;
}

Step 2: Write env vars into the container

Three variables per container. The API key is shared across all agents; the inbox ID and address are unique.

OPENMAIL_API_KEY=om_...
OPENMAIL_INBOX_ID=inb_8f3a1b2c
OPENMAIL_ADDRESS=jane@openmail.sh

For OpenClaw, set these in ~/.openclaw/.env or in ~/.openclaw/openclaw.json under skills.entries.openmail.env. For other frameworks, inject them into your agent’s environment (e.g. LangChain tool config, n8n node credentials).

Step 3: Add the skill file

This file teaches the agent how to send and receive email. OpenClaw loads it into the LLM’s system prompt; other frameworks use similar patterns (system prompts, tool descriptions). The agent calls the API via curl—no CLI binary needed. For OpenClaw, place at ~/.openclaw/skills/openmail/SKILL.md or pre-bake into your Docker image. For other frameworks, adapt the path—the skill content is the same.

SKILL.md

---
name: openmail
description: Send and receive email via OpenMail
requires:
  env:
    - OPENMAIL_API_KEY
    - OPENMAIL_INBOX_ID
    - OPENMAIL_ADDRESS
---

# OpenMail

Your email address is $OPENMAIL_ADDRESS. Use it when introducing yourself or sharing contact info.

## Send an email

```bash
curl -s -X POST "https://api.openmail.sh/v1/inboxes/$OPENMAIL_INBOX_ID/send" \
  -H "Authorization: Bearer $OPENMAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $(uuidgen)" \
  -d '{
    "to": "recipient@example.com",
    "subject": "Subject",
    "body": "Message body"
  }'
```

Save the `threadId` from the response to reply in the same thread later.

## Reply in a thread

```bash
curl -s -X POST "https://api.openmail.sh/v1/inboxes/$OPENMAIL_INBOX_ID/send" \
  -H "Authorization: Bearer $OPENMAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $(uuidgen)" \
  -d '{
    "to": "recipient@example.com",
    "subject": "Re: Original subject",
    "body": "Reply body",
    "threadId": "thr_..."
  }'
```

## Check for new messages

```bash
curl -s "https://api.openmail.sh/v1/inboxes/$OPENMAIL_INBOX_ID/messages?direction=inbound&limit=10" \
  -H "Authorization: Bearer $OPENMAIL_API_KEY"
```

## List threads

```bash
curl -s "https://api.openmail.sh/v1/inboxes/$OPENMAIL_INBOX_ID/threads?limit=10" \
  -H "Authorization: Bearer $OPENMAIL_API_KEY"
```

## Read a thread

```bash
curl -s "https://api.openmail.sh/v1/threads/{thread_id}/messages" \
  -H "Authorization: Bearer $OPENMAIL_API_KEY"
```

## Send with attachments

Use multipart/form-data to attach files:

```bash
curl -s -X POST "https://api.openmail.sh/v1/inboxes/$OPENMAIL_INBOX_ID/send" \
  -H "Authorization: Bearer $OPENMAIL_API_KEY" \
  -H "Idempotency-Key: $(uuidgen)" \
  -F "to=recipient@example.com" \
  -F "subject=Report attached" \
  -F "body=See the attached file." \
  -F "attachments=@/path/to/report.pdf"
```

Multiple files: repeat the `-F "attachments=@..."` flag. Max 25 MB total per email.

## Notes

- Always include `Idempotency-Key` when sending (prevents duplicates on retry)
- Use `threadId` when replying so the email threads correctly in the recipient's client
- For attachments, use `multipart/form-data` with `-F` flags instead of `-d` JSON
- `$OPENMAIL_*` variables are read from the container environment at runtime

Pre-bake into Docker - the skill file is identical for every agent. Only the env vars differ per container. For OpenClaw: COPY skills/openmail/SKILL.md /root/.openclaw/skills/openmail/SKILL.md

Step 4: Handle inbound email

Choose your delivery method. WebSockets are recommended for agents - no public URL needed, instant delivery. Webhooks work for traditional server-to-server integrations. The event payload is identical for both methods:

Event payload

{
  "event": "message.received",
  "event_id": "evt_7f2a3b4c",
  "inbox_id": "inb_8f3a1b2c",
  "thread_id": "thr_9d4e5f6a",
  "message": {
    "id": "msg_4c8d5e6f",
    "from": "customer@example.com",
    "to": "jane@openmail.sh",
    "subject": "Re: Your order",
    "body_text": "Thanks for following up...",
    "attachments": [
      {
        "filename": "receipt.pdf",
        "contentType": "application/pdf",
        "sizeBytes": 34210,
        "url": "https://api.openmail.sh/v1/attachments/msg_4c8d5e6f/receipt.pdf",
        "parsedText": "Order confirmation #1042\nTotal: $89.99",
        "extractionMethod": "pdf"
      }
    ],
    "received_at": "2026-02-24T10:05:00.000Z"
  }
}

WebSocket (recommended)
Webhook

Connect to wss://api.openmail.sh/v1/ws with your API key. No public URL, no signature verification needed - the connection itself is authenticated.

const WebSocket = require("ws");

const OPENMAIL_API_KEY = process.env.OPENMAIL_API_KEY;

const ws = new WebSocket("wss://api.openmail.sh/v1/ws", {
  headers: { Authorization: `Bearer ${OPENMAIL_API_KEY}` },
});

ws.on("open", () => {
  ws.send(JSON.stringify({ type: "subscribe" }));
});

ws.on("message", (data) => {
  const event = JSON.parse(data);
  if (event.event === "message.received") {
    const container = getContainerByInboxId(event.inbox_id);
    container.deliverEmail({ threadId: event.thread_id, message: event.message });
  }
});

ws.on("close", () => setTimeout(() => connect(), 5000));

Subscribe options:

{ "type": "subscribe" }
{ "type": "subscribe", "inbox_ids": ["inb_8f3a1b2c"] }
{ "type": "subscribe", "event_types": ["message.received"] }

Configure your webhook URL in the dashboard. OpenMail POSTs events to your endpoint with HMAC-SHA256 signatures.

const crypto = require("crypto");

app.post("/webhooks/openmail", (req, res) => {
  const signature = req.headers["x-signature"];
  const timestamp = req.headers["x-timestamp"];
  const expected = crypto
    .createHmac("sha256", process.env.OPENMAIL_WEBHOOK_SECRET)
    .update(`${timestamp}.${JSON.stringify(req.body)}`)
    .digest("hex");

  if (!crypto.timingSafeEqual(Buffer.from(signature, "hex"), Buffer.from(expected, "hex"))) {
    return res.status(401).send("Invalid signature");
  }

  const { event, inbox_id, thread_id, message } = req.body;
  if (event === "message.received") {
    const container = getContainerByInboxId(inbox_id);
    container.deliverEmail({ threadId: thread_id, message });
  }

  res.status(200).send("OK");
});

Always verify X-Signature before processing. See Webhooks guide for retry policy and deduplication via event_id.

Verify the integration

Create inbox - confirm OPENMAIL_INBOX_ID and OPENMAIL_ADDRESS appear in the container’s env
Send - tell the agent “send an email to your-test@email.com” - confirm it arrives
Inbound - reply to that email; confirm the event arrives over your WebSocket connection (or webhook) and routes to the container
Thread - tell the agent “reply to the last email” - confirm the reply threads correctly in the recipient’s client

Reference

What	Where	When
Skill file (SKILL.md)	Agent’s skills/config dir	Pre-baked in Docker or mounted at runtime
`OPENMAIL_API_KEY`	Agent env	Set once from your secrets
`OPENMAIL_INBOX_ID`	Agent env	Written when you create the inbox
`OPENMAIL_ADDRESS`	Agent env	Written when you create the inbox
WebSocket client or Webhook handler	Your backend	Connect to `wss://api.openmail.sh/v1/ws` or configure webhook URL in dashboard

Email deliverability

Warm-up schedules, inbox distribution, and content best practices to stay out of spam.

WebSockets

Real-time event streaming, subscribe protocol, reconnection.

Webhooks

Signature verification, payload format, retries.

API reference

Full endpoint documentation.

Get started

Core concepts

WebSockets

Webhooks

Guides

Integrations

Best practices

Resources

Security & privacy

Prerequisites

Step 1: Create an inbox

Step 2: Write env vars into the container

Step 3: Add the skill file

Step 4: Handle inbound email

Verify the integration

Reference

Email deliverability

WebSockets

Webhooks

API reference

Get started

Core concepts

WebSockets

Webhooks

Guides

Integrations

Best practices

Resources

Security & privacy

​Prerequisites

​Step 1: Create an inbox

​Step 2: Write env vars into the container

​Step 3: Add the skill file

​Step 4: Handle inbound email

​Verify the integration

​Reference

Email deliverability

WebSockets

Webhooks

API reference

Prerequisites

Step 1: Create an inbox

Step 2: Write env vars into the container

Step 3: Add the skill file

Step 4: Handle inbound email

Verify the integration

Reference