Hello Anpeter,
With the last version of Zammad 6.3.1-1722948970.e504ff79.bullseye I have finally been able to make use of the font configuration to apply on Outlook emails, before that the changes did not apply to outlook but other mail clients.
You can find how to apply this configuration here, but I think you already looked for it:
Try updating Zammad version and see if it works for Outlook. I have to say I am also using Outlook 365 and not 2016.