Request for Comments documents -- usually called RFCs -- give information about Internet email headers, transport, MIME messages, and much more. Although many people call RFCs "standards," few of them have been formally standardized. Still, RFCs are widely followed by Internet users. Internet drafts describe new methods that may become RFCs. Both of these are important places to get information, especially for a rapidly-changing topic like MIME.
You can get RFCs from at http://www.rfc-editor.org/rfc.html.
You can get Internet drafts from https://datatracker.ietf.org/public/idindex.cgi.