curl --request POST \
  --url https://app.sendmux.ai/api/v1/mailbox/messages/send \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "to": [
    {
      "email": "agent@example.com",
      "name": "Support"
    }
  ],
  "subject": "<string>"
}
'

{
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  },
  "data": {
    "message_id": "<string>"
  }
}