Microsoft Outlook app for mobile phones

Does anyone have any experience with deploying this application to their
user base? We have a few users who for whatever reason cannot authenticate
during initial setup, and a few others who cannot authenticate after their
active directory password has been changed. At first it was mostly android
users but now we are seeing the problem on iPhone 8's and even iPhone 6's
where it used to work until the mandatory password change. It's only a
handful of users so this is perplexing since we've uninstalled/reinstalled,
and included hard and soft device reboots. I've had my best people working
on it too (one is an MCSE fwiw), and we are out of ideas.

Thanks in advance for any input.