With iOS 10 and Exchange ActiveSync (EAS) version 16 and later, your Microsoft Exchange server synchronizes draft messages on your iPhone, iPad, and iPod touch.
The Drafts folder is at the top of the list of folders in Mail. In this folder, you can find draft messages that you've created and saved. When you use your Exchange account, you can edit and send these drafts from any of your devices or computers.
iOS 9 supports Exchange ActiveSync (EAS) version 16, but doesn't include Draft message synchronization.