And the definition of "SHOULD" (from RFC 2119) is "This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course."

Having said that, I regret my original characterization of the Message-ID header as a "requirement" and have updated the blogpost to be fair to all sides.

Thank you for bringing this up.