No. Don't confuse a client access protocol (like POP3 or IMAP4) with what exists inside a mailbox. If you have an Exchange Online mailbox as part of your Office 365 subscription, the mailbox is available to many different clients using different protocols, including POP3, IMAP4, ActiveSync, MAPI, MAPI over HTTP, Exchange Web Services, and the Graph API. If you want to use the Focused Inbox feature, you need to access the mailbox with a client using a protocol that supports the feature. So, you need one of:
Outlook Web App (OWA)
Outlook desktop (click to run version) for Windows or Mac
Outlook for iOS or Android
If you want a mobile client, you are limited to running OWA in a browser on a mobile device or Outlook for iOS or Android.
Thank you so much for taking the time to reply to me. I have been searching around the internet for weeks trying to find a solution to this problem. Unfortunately, I am not much of a techy so a lot of that didn't make very much sense to me at all. Would it be possible for you to provide links or is there I could maybe get in touch with you directly to help me work through this?
Thanks. That solves the mystery of why it works on some devices and not others. The Windows 10 Mail client is evidently still awaiting an update that Outlook for Android already has. Unfortunately I prefer the built in mail client to Outlook.