I'm not very familiar with the details of sending email but I'm getting the runaround between a vendor whose system automatically generates outgoing emails with the content I have configured and the Outlook engineers at my organization who have said that Microsoft has told them the presence of non-ASCII characters in the email are causing DKIM to fail, and that sending emails in UNICODE format is a workaround.
Is one of these parties clearly wrong? What specifics would you need to know in order to determine that?