Aqua-Soft ForumsI ran into this problem tonight and found out that theres a lot of people on google who are frustrated by it as well.
To my understanding even with your 'Message Format' set to 'Rich Text' in Preferences>Composing If your email message font hasn't been manually edited, mail.app will send it in Plain Text format (completely ignoring the Helvetica 12 you type it in). The only way I found to get around this is to have a signature with a font alteration (anything besides Helvetica 12 so the html header in the email would be sent)
Does anyone have a better/easier way to make sure every message sent will be in Rich Text by default?