MacOS – How to set a default font for outgoing messages in Mail on Lion

Mail in OS X 10.7 (Lion) doesn't allow me to set a default font for outgoing messages, and it's finicky when I change it in the message.

When I show the font bar (by clicking the A button at the top right of the message window), the font bar doesn't retain its value if I set it before typing any text. If I change it and then start typing, it changes to what I set it as, but if I delete all my text then it reverts to default (Helvetica).

How do I get around this? Is there a plugin to solve this, is it considered a bug…? I found the Universal Mailer plugin, which is now open source.