Outlook 2016 frequently adds single quotation marks around email addresses when I send emails. There does not seem to be any consistent pattern to when or why. The same contact can either have quotation marks or not; the quotation marks are sometimes there when sending an original email, sometimes they appear when replying or forwarding. None of these contacts has quotation marks in the "Display As" field.
I use a CRM called Highrise and apparently this prevents my emails that are being forwarded to my CRM account from being recognized.