http://www.rfc-editor.org/rfc/rfc3297.txt
RFC 3297: Content Negotiation for Messaging Services based on Email, G. Klyne, R. Iwazaki, D. Crocker2002-07-01T23:00:00-00:00This memo describes a content negotiation mechanism for facsimile, voice and other messaging services that use Internet email. [STANDARDS-TRACK]