Content-Type: multipart/mixed; boundary="1"

--1
Content-Type: multipart/mixed; boundary="2"

--2
Content-Type: message/rfc822

Content-Type: text/plain

1
--2
Content-Type: message/rfc822

Content-Type: text/plain

22
--1
Content-Type: message/rfc822

Content-Type: text/plain

333

---- EXPECTED STRUCTURE ----
{
  "headers": {
    "content_type": {
      "ContentType": {
        "c_type": "multipart",
        "c_subtype": "mixed",
        "attributes": {
          "boundary": "1"
        }
      }
    }
  },
  "attachments": [
    {
      "Message": {
        "headers": {
          "content_type": {
            "ContentType": {
              "c_type": "text",
              "c_subtype": "plain"
            }
          }
        },
        "html_body": [
          {
            "contents": "<html><body>1</body></html>"
          }
        ],
        "text_body": [
          {
            "contents": "1"
          }
        ]
      }
    },
    {
      "Message": {
        "headers": {
          "content_type": {
            "ContentType": {
              "c_type": "text",
              "c_subtype": "plain"
            }
          }
        },
        "attachments": [
          {
            "Text": {
              "contents": "22\n--1\nContent-Type: message/rfc822\n\nContent-Type: text/plain\n\n333\n"
            }
          }
        ]
      }
    }
  ]
}