I wrote:

> It's interesting, though, that the way Re: is used in email today,
> it's associated strictly with replies; nobody uses "Re: ___"
> when they first compose an email message.  (In contrast to its
> use in formal memos.)  So its meaning in email is different from
> its former meaning, and one could speculate that the change in
> meaning was helped by the resemblance of "Re:" to "reply".
> 
> Whoever was looking at the RFC regarding "Re:" before, did it say
> for what purpose it might be used?  Did it specify replies?

To answer my own question, RFC 2822 says (as Dion Houston already noted
in part),

   The "Subject:" field is the most common [of the three informational
   fields] and contains a short string identifying the topic of the
   message.  When used in a reply, the field body MAY start with the
   string "Re: " (from the Latin "res", in the matter of) followed by
   the contents of the "Subject:" field body of the original message.
   If this is done, only one instance of the literal string "Re: " ought
   to be used since USE OF OTHER STRINGS or more than one instance can
   lead to undesirable consequences.
   [emphasis mine. Take that, AW! :-)]

So yes, RFC 2822 (April 2001) did specify using Re: for replies.
Incidentally, the earlier RFC 822 (Aug. 13, 1982) did not say that
explicitly, but contains an example where the Subject begins with "Re: "
and there is an In-Reply-To field.

OK, this is getting way off-topic.

Lars


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