it's probably because you normally wouldn't access OWA if you're on WP, you'd access the exchange server directly. MobileIron (never heard of it until now) seems to be a MDM, and it does support Windows Mobile (the site says mobile, not phone, not sure if it's a typo or WM6 my be the highest version it supports)
it "should" just work, but if MDM isn't properly setup (or if WM6 is the highest it supports) then there may not be much you can do without buying your network manager a few beers and having him check it out.