Applications :: Not Formatting Message Fonts When Sending Emails

In Mail preferences I have message font set to Verdana and in the 'Composing' pane I have ensured that messages are sent as Rich Text, and yet I have a persistent and frustrating problem with Mail.app failing to apply font formatting to outgoing messages. When composing new messages everything looks fine (i.e. text displays in Verdana), but Mail isn't applying the formatting to emails; looking at the raw source of sent messages, Mail seems to be sending Plain Text messages with no formatting applied:

Code:
Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
Content-Transfer-Encoding: 7bit
Mime-Version: 1.0 (Apple Message framework v936)
Further adding to my confusion, if I append a formatted signature (configured in Mail preferences) to messages, the signature IS correctly formatted (in Verdana, no less).....