Some Emails in Outlook 2016 will Convert to Chinese Characters after reading some after archiving using enterprise vault. We had this issue on a specific set of emails and not all all emails or all domains. Then we found that the issue with the specific exclaimer signature which was imported from a different instance which is a lower version of it. We deleted the exclaimer signature and recreated from scratch and issue never re appeared after that.
if your email is stamped or altered with any signature software . Try to remove it and see if issue appears to happen or not.