Since the latest update I have been getting an “Authentication Failed for Outlook Email” error message on my Samsung SG7 cellphone.
Do you have an “app password” set up for this account? MS appears to be switching accounts over to OAUTH2 on a rolling basis. Since K9 doesn’t support that you’ll need to use the MS facility to generate an app password and use it, instead of your normal site password, with K9 logins (receive and send).