[Users] Message-ID

[Rich Pieri]

On Thu, 31 Jan 2013 22:15:43 +0000
RW <rwmaillists at googlemail.com> wrote:

> It depends on where the rewrite occurs, there isn't any reference
> header in the first mail in a thread. Aside from threading by subject
> the only standard way to connect that to rest of the thread is by its
> Message-ID.

There is no standards compliant way to maintain thread coherency with
Internet mail. RFC 822 does not require anything usable be parsed from
In-Reply-To and References. RFC 2822 requires that both headers have
Message-ID strings in them. However, since it is perfectly valid for a
single message to have multiple different Message-ID fields (a single
reply to multiple messages, for example) you still cannot rely on
Message-ID to be a reliable threading mechanism. It's nice when it
works out but this is not required by mail handling standards. The only
thing that can be depended on according to RFC 2822 is that every
Message-ID generated by a mail system is unique.

This is quite different from Usenet (RFC 1036) which requires
Message-ID and References fields maintain thread coherency.

-- 
Rich P.