仕様書

BNF

 Message-Context = "Message-Context:" message-context-class
 message-context-class = ( "voice-message" 
                         / "fax-message" 
                         / "pager-message" 
                         / "multimedia-message" 
                         / "text-message" 
                         / "none" 
                         / extension-type ) 
 extension-type = token  ; Defined and registered type 
                / vnd.token ; Experimental, private use 
 token = mime-token	;; not starting with the characters "vnd."> 
 vnd.token = "vnd." token	;; Vendor-specific, private token

歴史

• urn:ietf:id:draft-ietf-vpim-hint-07
• [1] Message-Context: は RFC3458 になりました。
• [2] [IANAREG] Context labels for Internet Messages http://www.iana.org/assignments/message-header-types

[3] RFC 3938 Video-Message Message-Context: RFC 3458 から video-message が独立しました。

[5] rfc3938, 2021-06-22T04:07:47.000Z https://datatracker.ietf.org/doc/html/rfc3938