Sending custom HTML emails via Yahoo

I?ve been trying unsuccessfully to send a custom html email thru my yahoo account. I?m using the yahoo web based interface and have selected ?compose message as plain text? in the general preferences page. Then I paste a simple html code and send it to a different account. The other account do not format the html, it display it as I paste it. How can I do this? do I need a client based email program? I have Outlook 2000 but have not found how to do this in the help.