Okay, a bit more digging, removing and re-adding the account on my iPhone didn’t make any difference, but here is what did work to fix this:
- Go to the account settings
- Advanced
- turn SSL off
- try to use Mail client (doesn’t work)
- turn SSL on
- try to use Mail client (suddenly works again)
So, I’m not sure if that is a “permanent” fix, or if there is something else at play here, but my guess is that it is definitely SSL related?