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The Sendmux Sending API lets you send emails programmatically through your configured delivery providers. It supports single and batch sending, attachments, idempotency keys, and returns structured responses with per-message status.

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Base URL

All Sending API requests should be made to: 
https://smtp.sendmux.ai/api/v1
This is a different base URL from the Management API (app.sendmux.ai). The Sending API runs on the email delivery infrastructure for optimal performance.

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Authentication

Authenticate using an API key with the email.send permission. Pass it as a Bearer token in the Authorization header. 
curl -X POST https://smtp.sendmux.ai/api/v1/emails/send \
  -H "Authorization: Bearer smx_your_key_here" \
  -H "Content-Type: application/json" \
  -d '{
    "from": { "email": "hello@yourdomain.com", "name": "Your App" },
    "to": { "email": "user@example.com" },
    "subject": "Welcome!",
    "html_body": "<h1>Welcome to our platform</h1>"
  }'
API keys are scoped to a team. Create and manage keys under Settings > API Keys in the dashboard. Keys used for sending require the email.send permission — the mailbox key category includes this by default.

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Permissions

EndpointRequired permission
POST /api/v1/emails/sendemail.send
POST /api/v1/emails/send/batchemail.send
GET /api/v1/openapi.jsonNone (public)

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Response format

All responses use the same JSON envelope as the Management API.

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Success

{
  "ok": true,
  "data": {
    "message_id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
    "status": "queued"
  },
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  }
}

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Error

{
  "ok": false,
  "error": {
    "code": "insufficient_credits",
    "message": "Your team's balance is too low to send this email.",
    "doc_url": "https://docs.sendmux.ai/sending-api/errors#insufficient_credits"
  },
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  }
}

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Rate limiting

Each sending API key is rate-limited to 1800 requests per minute (batch requests count as 1). Rate limit headers are included on every response: 
X-RateLimit-Limit: 1800
X-RateLimit-Remaining: 1794
X-RateLimit-Reset: 1679313700
When the limit is exceeded, the API returns a 429 status with a Retry-After header.

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Conventions

  • snake_case fields in all JSON request and response bodies
  • UTC ISO 8601 timestamps in RFC 3339 format: 2026-03-19T10:30:00Z
  • Public IDs only — internal database IDs are never exposed
  • Cache-Control: no-store on all authenticated responses (the OpenAPI spec endpoint uses public, max-age=3600)
  • JSON only — Content-Type: application/json

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OpenAPI specification

The full OpenAPI 3.1 specification is available at: 
https://smtp.sendmux.ai/api/v1/openapi.json