GiveUp Blacklist

Hmm, good point. Does this count as a soft error and get blacklisted after X times based on the settings in backend? That would make sense.
Keep in mind, per RFC, a mail server does not need an MX record, it will default to the A/AAAA record if need be.
Hence, "no mx" does not need to be classed as blacklist/bounce, just as certain MTAs have an optional setting for this classification, to allow choice. I would recommend to let it play out until it is a real bounce after a few days.