> Arguments like "twenty-four hours is short enough to not cause serious disruption of legitimate traffic" and "we already know that spam senders rarely use a fully compliant SMTP implementation to send their messages" are 20 years out of touch and completely void of connection with reality.

Just recently I found out a very prominent local service recovery emails are not delivered to the end-user mailbox.

Reason? The email doesn't have Message-ID. Like it get's generated, sent out, "my" PMG box receives it... and throws it out because no Message-ID. Insult to an injury? It was password recovery emails. Regular marketing ones are going through.