I have been using the email app on my Galaxy S5 for checking my Hotmail account for years. I noticed this morning that I was getting notifications that my authentication was failing for no apparent reason. I've tried to clear the Email cache, deleted the account entirely, made sure I could log in without any problems from other devices (PC, laptop, etc., no issues), but every time now I try to set my Hotmail account back up for my email it tells me "Unable to set up account - Authentication failed". I am absolutely using the correct authentication (username, password) so I assume it's something wonky within the app that's causing the issue.

This happened to me one other time a couple of years back but I don't remember what I did to fix it