I fully expect that all our experts know this but it may be news to others

Ron Sommer wrote:Login to your outlook.com account using a browser. Does it say Outlook.com or Outlook Mail (Preview)?
KB3093594 should only be installed if you have an outlook.com account that has been upgraded to Outlook Mail (Preview).
If you have more than one outlook.com account, you will have to decide if you want the accounts that have not been upgraded to Outlook Mail (Preview) to work or do you want the accounts that have been upgraded to work.
No Outlook Mail (Preview) accounts - do not install KB3093594.
An account has upgraded to Outlook Mail (Preview) - install KB3093594.