> Also, the author was able to get their mail delivered to a personal gmail.com address. The issue was with a Google Workspace custom email domain. This further makes me think of this as a security/spam related issue. Google is clearly capable of processing the message without a Message-id, they are just refusing for business customers.

That's the annoying part to me.

An email is an email. By applying different rules for rejection on different mailboxes, gmail creates a system where it's harder for would-be implementers to test compliance.

If tomorrow gmail creates a new type of mailbox, will there be a third set of rules to have your message delivered?

In my experience, email is an unreliable way to communicate any time-bounded critical information. When I want to be sure an email was transmitted on either side, the only reliable way to ensure this is to use a distinct channel to validate reception and confirm content.

That is, when some hotline tell me that they just sent and email with the information, I ensure they hold the line until I got the actual email and checked it delivers the relevant information to fulfill the intended process. And when I want to make sure an email was received, I call the person and ask to check, waiting until confirmation.

It’s not that much SMTP/IMAP per se as the whole ecosystem. People can legitimately get fatigue of "is it in my junk directory", "it might be relayed but only after the overloaded spam/junk analyzer accept it", or whatever can go wrong in the MUA, MSA, MTA, MX, MDA chain. And of course people can simply lie, and pretend the email was sent/received when they couldn’t bother less with the actual delivery status.

There are of course many cases where emails is fantastic.

At scale, it's very hard to distinguish malicious intent from the simple consequence of being the largest operator in a space so any motion one makes makes waves.

For what it's worth: having seen some of how the sausage is made, Google isn't particularly interested in screwing over a small reputable German provider. But they also aren't particularly interested in supporting such a provider's desire to route messages to their users because the provider is small. At their scale, "I've forgotten how to count that low" is a real effect. And email, as a protocol, has been pretty busted for decades; it's not Google that made the protocol so open-ended and messy to implement in a way that providers will agree is correct.

> Nothing of significance ever happened when Google had large optics fiber connections with NSA installed illegally and claimed to have no knowledge of it

Nothing of significance outside Google. Inside, Google initiated a technical lift that turned their intranet into an untrusted-by-default ecosystem so that data was encrypted on the fiber (as well as between machines within a datacenter, to head off future compromised-employee attacks). That process took at least five years; I suppose there's a scenario where it was all smoke and mirrors, but being on the inside in the middle of the process, I watched several C-suite who are not particularly good actors be bloody pissed at the US government for putting itself into Google's "threat actor" box and making that much work for the system engineering teams.

Also, an engineer at Google then made an end-to-end email crypto plugin for Chrome, including a flag that was a nod-and-middle-finger to the information revealed in the Snowden documents. https://techcrunch.com/2014/06/04/nsa-mocking-easter-egg-fou...

Long time ago when I was managing ISP email relay and customers asked "Where is the message I've sent?" seeing in the logs message accepted by receiving SMTP server was the end of the debug for me: I just handed the customer the part of the log and suggested talking to the receiving side IT administrator.