> Well, [google] does not manage usenet groups and mailing lists.
They do. Sort of.
Google used to nntp, and manages the largest usenet archive; They still have one of the largest mailing list servers in the world, and they still perform distribution on those lists via SMTP email.
They still have all of the problems associated with it, as do lots of other mail/news/list sites still do that are a fraction of Google's size.
> Delivery status notifications are considered best effort so it wouldn't make sense to block messages for that case.
Sure it does.
You consider them best-effort, but that doesn't follow that I should consider them best-effort. For a simple example: Consider spam.
In any event, if you keep sending me the same message without any evidence you can handle them, I'm not going to accept your messages either, because I don't know what else you aren't doing. That's part of the subtext of "SHOULD".
Most big sites take this policy because it is internal nodes that will generate the delivery notification, but the edge nodes that are tasked with preventing loops. If the edge node adds a Message-ID based on the content, it'll waste CPU and possibly deny service; If the edge node naively adds a Message-ID like an MSA, the origin won't recognise it, and forwarded messages can loop or (if sent to a mailing list) be amplified. There also are other specific documented requirements related to Internet Mail that edge nodes not do this (e.g. RFC2821 § 6.3).
However you seem to be assuming Google is blocking messages "for this case" which is a little presumptuous. Google is presumably trying to save themselves a headache of handling errors for people who aren't prepared to do anything about it, the most common of which is spam. And the use of Message-ID in this application is documented at least as early as RFC2635.
> Additionally, Gmail adds its own message identifier on every message (g-msgid) because it knows that message ids can not be trusted to be unique.
Without knowing what Google does with the g-msgid header, you are making a mistake to assume it is equivalent to the Message-ID header just because it has a similar name. You have no reason to believe this is true.
> Finally just calling me ignorant is the cherry on top – please try to keep things civil on here.
I am sorry you are offended to not know things, but you do not know this thing, and your characterising my actions will make it very difficult for you to learn something new and not be so ignorant in the future.
Think hard exactly about what you want to happen here: Do you want Google (et al) to do something different? Do you want me to agree Google should? Who exactly are you trying to convince of what, and to what end?
I am trying to tell you how to interpret the documentation of the Internet, in this case to be successful sending email. That's it.
I am not likely to try and tell Google what to do in this case because of my own experiences running mail servers over the last 30 years, but listen: I am willing to be convinced. That's all I can do.
If it's something else, I'm sorry I just don't understand.