Strange, I don't know what to suggest at this point. I've tested 7.2, 7.3, 7.4, and 8.0.25 now all working fine with J3 and J4. Will try 8.1 sometime this week. I'm not seeing any issues with the emails. My guess is it's a problem with your mail server or a problem with your PHP 8 configuration. Maybe try a newer version of PHP 8? Possible it's just an early PHP 8 bug.