> 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.