I'm currently finding myself in a bad situation. A customer has Outlook 2013 and is connecting via IMAP to RackSpace email (non Exchange). After their recent upgrade to Windows 10, they discovered that IMAP wasn't syncing things correctly back to RackSpace. They lost all of their sent mail and several people lost random stuff from within folders. It's a very bizarre situation.

They are looking to leave RackSpace now and move to Hosted Exchange/O365.