HEADER     ([RFC5322] header of the message)
  TEXT       ([RFC5322] text body of the message) MULTIPART/MIXED
  1          TEXT/PLAIN
  2          APPLICATION/OCTET-STREAM
  3          MESSAGE/RFC822
  3.HEADER   ([RFC5322] header of the message)
  3.TEXT     ([RFC5322] text body of the message) MULTIPART/MIXED
  3.1        TEXT/PLAIN
  3.2        APPLICATION/OCTET-STREAM
  4          MULTIPART/MIXED
  4.1        IMAGE/GIF
  4.1.MIME   ([MIME-IMB] header for the IMAGE/GIF)
  4.2        MESSAGE/RFC822
  4.2.HEADER ([RFC5322] header of the message)
  4.2.TEXT   ([RFC5322] text body of the message) MULTIPART/MIXED
  4.2.1      TEXT/PLAIN
  4.2.2      MULTIPART/ALTERNATIVE
  4.2.2.1    TEXT/PLAIN
  4.2.2.2    TEXT/RICHTEXT