One of the available open-source libraries which does it all is OpenPop.NET:
http://sourceforge.net/projects/hpop[
^].
Don't be confused: it may seem that the library is about POP3 protocol, this is not so. Yes, client-side POP3 protocol implementation is available, but this is not the most important part of it. The project can be used as a detailed references on all kinds of standards/RFCs on MIME types, formats, everything which is needed for comprehensive parsing of an e-mail message.
—SA