I asked our InfoSec, gave them scenario if I encrypt the phone keep e-mails there on the phone why cant the policy via MDM be relaxed. They refuse as I could save an e-mail attachment on the SD card (unencrypted) breaching policy.
They then asked why don't I use a 3rd party tool/app to do it.. further research showed this is futile also, as the phone wont recognise it , not will the MobileIron MDM hence back to square one.
Comments at bottom this this article also show it cannot be done via PC either if I tool SD card out & used bitlocker on PC,
How to enable device encryption on a phone with Windows 10 Mobile | Windows Central