> 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?