curl --request GET \
  --url https://app.sendmux.ai/api/v1/mailbox/messages/{message_id}/attachments/{attachment_id} \
  --header 'Authorization: Bearer <token>'

{
  "error": {
    "code": "invalid_parameter",
    "message": "<string>",
    "retryable": false,
    "param": "<string>",
    "doc_url": "<string>",
    "errors": [
      {
        "field": "recipient.email",
        "code": "invalid_string",
        "message": "<string>"
      }
    ]
  },
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  }
}