Email syncing on WiFi only

I have a user with a new Nexus 5X. It is configured to sync his personal (Gmail) email and work (Exchange) email (using the Gmail app). Gmail syncs fine; however, Exchange will only sync when connected to WiFi. Even a manual sync will not download Exchange email. I have installed all updates (he is on Android 6.0.1, security patch level March 1, 2016). I confirmed that "restrict background data" is NOT enabled for anything. Turned off battery optimization (Doze) for Exchange Services & Gmail. I've tried various sync settings for the Exchange account (Push, 15 min, 30 min, etc). No luck. Exchange email will not download while on cell service, but Gmail will. As soon as I have a WiFi connection, the Exchange email downloads. Not sure what to try next; any ideas?