Microsoft have changed something, and here’s how to fix it.
Traditionally, Microsoft have used a system called “Basic Authentication” to allow you to configure their mailboxes on systems such as Apple’s iOS Mail app. However, they are now rolling out a change to “Modern Authentication” and it’s prompting many people to have to re-configure their device in order to carry on using their mailbox in the iOS Mail app.
Symptom: You will be prompted repeatedly for the password for your Microsoft Office 365 account. Even though you enter it, the prompt keeps coming back and your iOS Mail app doesn’t update.
Cause: Microsoft has disabled Basic Authentication in Office 365.