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@b.com and c@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@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@d.net is a (simplistic) mailinglist, however, AFAICT it is still considered proper that the copy it sends to e@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