http://www.rfc-editor.org/rfc/rfc1154.txt
RFC 1154: Encoding header field for internet messages, D. Robinson, R. Ullmann1990-04-01T23:00:00-00:00This RFC proposes an elective experimental Encoding header field to permit the mailing of multi-part, multi-structured messages. The use of Encoding updates RFC 1049 (Content-Type), and is a suggested update to RFCs 1113, 1114, and 1115 (Privacy Enhancement).