Ahh - STOP PRESS
It works!
I have a feeling it might have something to do with unticking all those email settings, saving them and re-ticking them all though? That’s what I just did after about 5 new post attempts.
I’m going to check this with another account…
EDIT - yes, this is the way to fix users who were created in this purgatory period. Untick all email settings, save, then retick all email settings and it works again.