By looking at the raw source view of a message, you can also see that no default font gets sent. Compare the raw source from a Mail message and that of an Outlook message -- check right after where it says . Also look at your received messages. You will notice that messages sent from non-Macs have not changed, or only the font face has changed, not the font size. Messages from other Macs, and messages you have sent, will change both font face and size. Your default font will be used if the font set is not recognized. Haven't you noticed that all people that send you mail from other Macs use your default font?
To send font information with your outbound messages, you must set each message's font by changing it within each message. If you don't, no font information is sent.
[robg adds: I hadn't noticed this because I never use rich text email, and had always assumed that the Message Font field was there to control how my messages looked to me, not to others.]

