A reminder to myself, and anyone else who may encounter this issue. I have occasionally run across it and finally nailed down the symptom and work-around solution... The problem is that in rare (maybe once in six months or so) I would get an error while syncing my mail in Windows Phone. It's an Exchange server, getting at the mail through OWA, and the fact that the error appeared & disappeared without warning, and only affected me, made it very mysterious.
To make a long story short, sent a message to a fictitious address on my own email server trying to test something, and naturally got a administrator's "non-deliverable address" message back. This NDA message in my in-box was causing the error code 8500201F, making my phone fail to sync. I discovered this by cleaning out my in-box, which got sync working again. I then started putting messages back a few at a time until sync failed again. I eventually narrowed it down to that NDA message. Not sure why it's a problem, but that's what it was.