rfc2822 April 2001

4.5.4. Obsolete identification fields

   The obsolete "In-Reply-To:" and "References:" fields differ from the
   current syntax in that they allow phrase (words or quoted strings) to
   appear.  The obsolete forms of the left and right sides of msg-id
   allow interspersed CFWS, making them syntactically identical to
   local-part and domain respectively.

obs-message-id  =       "Message-ID" *WSP ":" msg-id CRLF

obs-in-reply-to =       "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF

obs-references  =       "References" *WSP ":" *(phrase / msg-id) CRLF

obs-id-left     =       local-part

obs-id-right    =       domain

   For purposes of interpretation, the phrases in the "In-Reply-To:" and
   "References:" fields are ignored.

   Semantically, none of the optional CFWS surrounding the local-part
   and the domain are part of the obs-id-left and obs-id-right
   respectively.