Maybe you should start digging in some RFCs. I suggest:

RFC 822, "STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES", https://www.rfc-editor.org/rfc/rfc822

RFC 2047, "MIME (Multipurpose Internet Mail Extensions) Part Three Message Header Extensions for Non-ASCII Text", https://www.rfc-editor.org/rfc/rfc2047, and especially the 4.2. section about "The "Q" encoding" which I believe should details the "special characters" like =92, =93, etc...