Like many others, my email notifications stopped when I upgraded to 2.3.1. I tried the no-sender.php plugin, and that gave me partial emails; the Subject and Content were both blank, but the rest came through. Then I tried adding a firstname.lastname@example.org email address, and, once I de-activated the no-sender plugin, that also gave me partial emails. (With both running, I didn't get anything.)
I am at a loss what to do about this. I have looked over pluggable.php and the mail function and everything seems to be in place. Does anyone have any ideas?