Non-unique Message ID in mail messages

[Jochen Bern]

On 27.01.22 16:17, Daniel Ryšlink wrote:
> RFC 5322 clearly states that mail messages SHOULD contain a Message ID 
> identifier, but if the do contain it, it MUST be globally unique.

The problem with that requirement being that it remains unclear how long 
the mail( copie)s it's attached to remain interchangeable versions of a 
"globally unique" message.

When an e-mail sent to a at b.com and c at d.net gets split into two copies 
and the separate mailservers for b.com and d.net each forward a copy *as 
is* to e at f.net, the f.net server would be entirely correct to call 
whichever arrives second a duplicate, even though they'll differ by at 
least the Received: headers.

When c at d.net is a (simplistic) mailinglist, however, AFAICT it is still 
considered proper that the copy it sends to e at f.net retains the original 
Message-ID - even though there will be more extensive changes to the 
headers (list headers, Reply-To:, possibly stuff like retrofitting SPF 
and DKIM, ...). Assuming that the list mail arrives second at f.net, 
deduplication will keep the recipient from ever reaping the benefit of 
those changed headers (as in, having a "Reply List" button pop up in TB).

(However, if I understand correctly that on the list you're talking 
about you see the same Message-ID applied to e-mails that are 
essentially *replies*/followups to the original one with entirely 
different content, I suppose that most people will agree that they 
*should* each have a Message-ID of their own, with the IDs of the 
earlier e-mails appearing in In-Reply-To: and References: headers to 
support threading in MUAs.)

Kind regards,
-- 
Jochen Bern
Systemingenieur

Binect GmbH
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