On 10/19/2017 12:37 AM, Stephen J. Turnbull wrote:
The IETF has NO position on WHEN this should be done because it's not relevant to interoperability. My personal reasoning with respect to mailing list managers like Mailman which normally pass through all text/plain, and perhaps add some tags to Subject and prefix or suffix the body, is that users (including posters) would be quite annoyed if de-duping didn't work. And those of us who deal with mail in sophisticated ways would be quite upset if the Message-ID we give it doesn't correspond to the Message-ID distributed by the list and in the archive.
I believe RFC 6377 makes it fairly clear if a message is new or not. TL;DR: If anything other than the SMTP envelope is modified, then the MLM is a resending MLM, which necessitates a new message with a new author and Message-ID.
I can respect your concern about the Message-ID changing, especially with deduplication. However, I counter that the new message from the resending MLM is in fact a different message than the one that the original author sent to the resending MLM. So, if you were in the To / CC / BCC of the message from the original author, you /should/ receive two copies of the message.
Fortunately nicer MLMs, like Mailman, can detect that a list subscriber was included in the To or CC and act on the subscriber's configured option if they want to receive a copy of the message from the MLM that they received directly.
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