How do I change the default font so that when I reply to an email it automatically sets it to Calibri instead of Times New Roman.

This works sometimes but not always.

Also when I add an email signature it changes the font to Times new roman, even when I specify it to be Calibri, which is terrible.

Thanks
I am working with Outlook 2011 (latest version and update) with Mountain Lion.
All my email are in HTML with Calibri 15.
Sometimes, not always, the recipient receives only a few rows with a different dimension.

Instead, my signature with Calibri 12, when someone answer me, the signature is changed to 13.

Why is that?

I have been trying, unsuccessfully, to change the default font for messages in the Preferences of Outlook 2011. Is there some secret? I change it. Close the window. Reopen the window and go to the Fonts tab for Preferences. It's back to Calibri. I've tried changing it, quitting Outlook, relaunching Outlook, and checking the Preferences, only to find that it's reverted to Calibri. I've changed other Preferences, but this one won't take. Please help.