I'm a recent Microsoft hire. That being said, it's obvious that they're using Office 365's Hosted Exchange for their email, which is a departure from G Suite at NYU and CacheCash, and my personal FreeBSD/Postfix/Dovecot setup.
Like many big companies, Microsoft requires 2FA (Two-Factor Authentication) for logging in to company resources. Well, okay, I done this before. But then, to use corporate email “officially”, I have to use not only Outlook, but also the InTune App, which “manages” my device.
I can understand InTune for most users, but for me, reasons including privacy and the fact that I rooted my device (Google Pixel 3), I did not want MS to “manage” my device.
However, there is one way you can get your Office 365 Mail on Android without Outlook or InTune: Just use Firefox as “Outlook”, and log in to Office 365 in “desktop mode”. The only thing you lose is notifications, but hey, it's not like K-9 Mail is great with that either with my personal email setup.
So how would you do this?
First, if you don't already have it, download Firefox from the Play Store, or externally if you “ungoogled” your Android.
Then, launch Firefox, and go to office.com.
Next, touch on the three dots and select Request Desktop Site. You need to do this, otherwise you would need InTune.
Zoom in and select Sign In.
Sign in the way you normally would, I won't describe it here.
Then, when it asks you to save your information:
If you want to emulate the official “Outlook” app and can live with cookies, say Yes
If you absolutely don't want cookies and can live with relogging in anytime, say No
I chose Yes, but think for yourself whether you want privacy or convenience.
Then, when you want to use your mail, use outlook.office365.com in the address bar. Consider pinning it to the Firefox start screen.
Keep in mind that these instructions may work on other browsers, but I have not tested them here.