great, error codes! 451 4.7.1 Try again later in postfix can be a few things, but my guess is 1. on graylisting or graylisting without the good parts, I’ve noticed its use with murena mail before and I’m just not a fan of GL, but I never ran a freemailer, 2. something in postfix plumbing erroring out - or plain simple things as disk space - so the queue isn’t processing.
As to graylisting, when I compare delayed mail (same sender - hivemq - same content - signup mail) to the one quick to arrive, the BAYES_SPAM header isn’t present with the latter. As it hit the same mail server and is of the same content, not sure what’s the heuristics that decides what gets the bayes treatment. It clearly isn’t working well for time critical mail.
It is not the DANE failure that the thread was on up to that point. I’d split it of adding “(try again later)” to the title, and add “(failed DANE validation)” to the original.