Correction to my initial post.
Please disregard the mention of MAPI.
The account is setup and recognized as Exchange by the phone (iOS 11).
I followed a tutorial regarding Exchange Drafts syncing to the server (https://blog.appriver.com/2016/11/ios-10-introduces-draft-folder-synchronization -in-mail-app/).
Mail/Accounts/Exchange/Account/Advanced Settings/'Tap the dial next to Store Drafts on Server to slide it to the right'.
The above link references iOS 10.
On my phone (iOS 11): Account and Passwords/Exchange/Account/Advanced Settings/'There is No Drafts option'.
Maybe as of 11 the save drafts option is no longer optional and is supposed to just do it?
Mark