Some mail accounts not surviving iOS10 upgrade

Started rolling out the upgrade and am finding that some accounts, specifically pre-outlook hotmail.com accounts, are not surviving the upgrade. iOS prompts for credentials to be entered, which usually fails. Then on the second round, it removes the / in the hotmail.com/username@hotmail.com box, which appears malformed to begin with, and the / is missing from the OSK. Sometimes it does appear to work, until you get to an "allow access" screen, then it throws an "address mismatch" error.

Can these accounts be saved?

A scorched earth rebuild of the account gets mail flowing, but thats way more PITA than I bargained for on the upgrade and I have a boatload of devices in the queue.