Email – send HTML question

On a Windows XP Pro system, when an email from Outlook Express is sent, it pops up a message that says: “You are sending this message as rich-tect (HTML) message. Some of the recipients prefer to recieve only plain-text messages. How would you like to send this message?” and has three buttons: “send HTML”, Send Plain Text” and “Cancel”. I have went into tools, options, send and manually selected plain text. It still gives the message.
Any suggestions?

Thanks