Introduction

https://app.lettr.com/api/

curl https://app.lettr.com/api/emails \
  -H "Authorization: Bearer lttr_xxxxxxxxxxxx"

curl -X POST https://app.lettr.com/api/emails \
  -H "Authorization: Bearer lttr_xxxxxxxxxxxx" \
  -H "Content-Type: application/json" \
  -d '{"from": "you@yourdomain.com", "to": ["recipient@example.com"], "subject": "Hello", "html": "<p>Hello!</p>"}'

{
  "message": "Email queued for delivery.",
  "data": {
    "request_id": "12345678901234567890",
    "accepted": 1,
    "rejected": 0
  }
}

{
  "message": "The sender domain is not configured or approved for sending.",
  "error_code": "unconfigured_domain"
}

{
  "message": "Validation failed.",
  "error_code": "validation_error",
  "errors": {
    "from": ["The sender email address is required."],
    "to": ["At least one recipient email address is required."]
  }
}

{
  "headers": {
    "X-My-Header": "custom-value",
    "X-Campaign-ID": "spring-2026"
  }
}