Dear Liman,
Am I right in understanding it as "my host doesn't know its own hostname (identity), so it's unable to create a useful Message-ID"?
Not sure, because my original problem was that Emacs or one of the components which intervene *locally* generate and introduce the Message-ID no matter the value of message-required-mail-headers. This is still unresolved. The underlying intellectual conundrum I think you summarize well:
"Which entity (i.e., which device) should be responsible for adding the Message-ID header to a message composed on a device which has little or no notion of its own host identity, and in those cases what should the Message-ID be?"
My device composes the message before posteo.de transmits it. What is my authority to use the formula x...@posteo.de in the Message-ID, especially because the mix of self-generated xxxx and official-looking @posteo.de looks intellectually confusing. Think of physical post: the post office will stamp the branch number and timestamp the envelope when they accept my parcel. posteo.de are charged with transmitting my email: it only sounds reasonable they would add the Message-ID, which is essentially hostname plus timestamp. Now one could make the case for the added freedom which comes with deciding the Message-ID of your own emails: but if the host can overwrite this anyway, then the benefit of the potential added privacy which comes from setting Message-ID at the MUA level is not guaranteed, but the drawback of having possibly same Message-ID's for different emails (sent through the same host or even distinct hosts) is real.
I happen to know the author of the mail specification RFC, and I'm thinking about asking him for his thoughts. Would you object to that?
I don't object.
