Thunderbird / Outlook: default HTML formatting not "sticking" / showing in webmail?!

The software:

- Thunderbird 2.0.0.9
- Outlook 2007

Domains I have tested this with:

- GMail
- Yahoo

The problem:

The outgoing text format set in both email clients is Arial font, smallish (10 pt), and blue. This is set in each program's options rather than on a per-message basis. "HTML" is specified as the outgoing format where applicable in each client.

When sending to two of my own email addresses and checking them in webmail instead of a client, the text is Arial / medium (12 pt) / black.

However, the formatting sticks if I manually specify it in each message as I type it, but that's just silly. Also, the formatting info is there, because when the messages get retrieved via POP, they show correctly in either client. Additionally, if specifying "Rich Text" for outgoing messages in Outlook, the formatting works as it should.

Any thoughts on why this is and how I might be able to fix it?