Using Outlook for Android in business environments for email and scheduling
Outlook Mobile Support (for me, on Android) insisted it was not Outlook. I ended up going to O365 Support using my Microsoft Complete support Plan, purchased with my Surface Duo. So, this may not be at-all relevant to others here. I was asked to uninstall and re-install the app. Once done, setting up the IMAP account resulted in something new.
After entering the settings, we encountered Login Error. The mail server certificate is invalid, or something to that effect. I could cancel setup or continue to login. Continuing to login results in Downloading Your Email that, overnight, would not complete.
I googled 'email server certificate'. It appears to have nothing to do we us the users, and our setup information. The certificate appears to me something that Microsoft has to register, I assume with some industry accepted registrar and that the email service provider must refer to to ensure secure connections. So, I have asked my service provider to double check whether or not they are referencing the latest list of certificates. I have advised Outlook Mobile Support of this and an awaiting a reply.