Editing an outlook message on the iPhone
To track edits to a document, save it in OneDrive and then edit it it in Word or OneNote or Excel or Powerpoint. Outlook doesn't do revision tracking on iOS or macOS in any current 2016 or 2019 version of the suite. I don't even thing outlook web app even tracks versions, and that's where they roll out most features first now.

Then send a link to the document or just share it. We also use box and iCloud for collaborative editing to track changes and you might also consider Google if that's open for your group.