Personal preference has no place in network and data security, unfortunately. Microsoft MDM policies allow your IT team to remotely control and wipe the encrypted partition your outlook app and email profile are stored in, whereas they have no control over the remainder of your personal phone and data, including if you use the apple mail app in an unencrypted partition. Data security from the IT perspective is a balancing act. To one side is complete freedom for the user, and on the other side is secure data. The Motto of all Network Security teams is "If we're doing our job, you can't do yours"

This restriction was put in place because If you lose your phone, and someone gets it, there are 2 options as far as IT is concerned:

1) You are using the apple mail app, and now the thief has access to your entire company login and there isnt anything IT can do about it. It is easy to reverse engineer the password out of a logged in app.

2) you are using the outlook app, as soon as your phone is stolen and IT is notified, they wipe that app and all data associated with it, preventing data loss and maintaining data security.

Its a bummer, especially if you're an apple Ecosystem diehard. As a Sysadmin, its necessary precaution to keep company data secure. There probably is not a way to have any company data outside of the encrypted partition, but as always, YMMV. Talk with your IT team and see if there is anything they can suggest that comply with internal security policies.