Hi All,
I had the same issue this morning.
A Google search lead me here:
https://superuser.com/questions/732299/how-to-change-the-body-font-in-outlook
This thread is 10 years old but it worked for me to change back to Calibri.
Close Outlook and any other Office programs before you do this or you will get a "open as read only" message when you try to edit.
Go to C:\Users(your username here)\AppData\Roaming\Microsoft\Templates
AppData is a hidden file so you will have to show hidden items in the view menu.
View>Show>Hidden Items
Right-click the NormailEmail.dotm template and click Open (not New).
Go to the Design tab, click the Fonts button/dropdown and blick to Customize Fonts.
There you can change the default Body font to whatever you want. Note that you're only changing the typeface itself, not color, size etc.

I changed the font back to Calibri and saved the changes as +Body (the default was Custom1 in the name field).
Opened Outlook and my +Body font is fixed.
I hope this helps.
Best Regards,
Mark Nabb