Changing default font

Changing default font

Hi

Im looking for a way to set a default font for every user in either the team or per Inbox. It currently defaults to Serif however we are wanting to change it to Arial. I am not looking to change how it looks for the user (which i am aware you can change in the settings and personlisation options on the top right), instead the default font when sending a email.

Hope you can help
Thanks
Jonathan