Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!mit-eddie!killer!wisner
From: wisner@killer.DALLAS.TX.US (Bill Wisner)
Newsgroups: comp.mail.mh
Subject: Re: In-Reply-To header field
Message-ID: <5886@killer.DALLAS.TX.US>
Date: 23 Oct 88 18:02:52 GMT
References: <8810230616.AA15417@odin.think.com>
Reply-To: wisner@killer.Dallas.TX.US (Bill Wisner)
Organization: The Big Bang Burger Bar
Lines: 10

The appropriate portions of RFC822:

optional-field =
            /  "In-Reply-To"       ":"  *(phrase / msg-id)
phrase      =  1*word                       ; Sequence of words
word        =  atom / quoted-string
atom        =  1*<any CHAR except specials, SPACE and CTLs>

An in-reply-to field can, therefore, consist of just about anything,
if you interpret it as being a string of atoms.