Knowing Imap Message Part Numbers

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i am using visual C#.
i have some body structure of the form

BODYSTRUCTURE ((("TEXT" "PLAIN" ("CHARSET" "utf-8") NIL NIL "QUOTED-PRINTABLE" 717 9 NIL NIL NIL)(("TEXT" "HTML" ("CHARSET" "utf-8") NIL NIL "QUOTED-PRINTABLE" 3126 40 NIL NIL NIL)("IMAGE" "PNG" ("NAME" "image.png") "<1.3499534900@web193301.mail.sg3.yahoo.com>" NIL "BASE64" 494712 NIL NIL NIL) "RELATED" ("BOUNDARY" "-1280599187-776620704-1335962964=:671") NIL NIL) "ALTERNATIVE" ("BOUNDARY" "-1280599187-319159910-1335962964=:671") NIL NIL)("TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME" "imap") NIL NIL "BASE64" 53416 684 NIL ("ATTACHMENT" ("FILENAME" "Imap.txt")) NIL)("IMAGE" "JPEG" ("NAME" "img72.jpg") NIL NIL "BASE64" 1462716 NIL ("ATTACHMENT" ("FILENAME" "img72.jpg")) NIL)("APPLICATION" "PDF" ("NAME" "April") NIL NIL "BASE64" 46294 NIL ("ATTACHMENT" ("FILENAME" "April 2012.pdf")) NIL)("AUDIO" "MPEG" ("NAME" "audio") NIL NIL "BASE64" 189560 NIL ("ATTACHMENT" ("FILENAME" "audio.mp3")) NIL) "MIXED" ("BOUNDARY" "-1280599187-1454468470-1335962964=:671") NIL NIL)

how can i know the various part numbers.
i want to fetch only some parts of the message.

plz don't give reference to any imap client library.

i just want to know the syntax used in imap to do this.

does the server always returns the information regarding boundary in the bodystructure fetch?

thanks in advance

plz help