Fix default font so that no part of the message reverts to Calibri
Outlook allows you set a default font and choose your own font for your signature, but no matter what you do, there will always be at least 1 blank line below your signature (in new messages and replies) where the font reverts back to Calibri. This means if you remove your signature, then start typing, you are not in your default font and must manually change it.
Sicilia Vides commented
You can change that, in File/Options/Mail/Signatures/Signature of email address, there it displays some options where you can modify and adjust the Font, size, and any other information of your new and reply messages.