I have an iPhone SE. I use Outlook for iOS as my email client. I use Xfinity/Comcast email (IMAP). All was fine for months, then a message began popped up. It required me to sigh in. I would enter my credentials, only to be declined. A message telling me that either my user name or password was incorrect. After tons of research, deleting and reinstalling Outlook several times, and deleting and reinstalling my mail account several times, I found the fix. All I needed to do was deactivate Norton’s VPN within Norton 360 for iOS. It’s unfortunate, and I hope Norton is able to correct the problem with an update. I hope this helps at least one other person.