http://www.rfc-editor.org/rfc/rfc2112.txt
RFC 2112: The MIME Multipart/Related Content-type, E. Levinson1997-03-01T23:00:00-00:00The Multipart/Related content-type provides a common mechanism for representing objects that are aggregates of related MIME body parts. This document defines the Multipart/Related content-type and provides examples of its use. [STANDARDS-TRACK]