The update to V. 21.5 has re-introduced the Mail Shield bug which prevents the Windows Mail App syncing email accounts. Tested on two devices so far and both required disabling Mail Shield to restore functionality to the mail app.
Not good enough, avast.
Edit: Issue now also confirmed as present on two more devices, turning mail shield off resolves it.
Edit2: 18/06/21 - Updated to build 21.5.6354.675 and issue with Mail Shield is now resolved.