Wired's How-To Wiki points out that HTML emails rendered by Outlook are not only occasionally hard to format, they'll look different (and broken) in many clients. They follow up with tips on turning some or all emails to plain text.
If you're generally emailing inside a closed circle with Outlook users, HTML-formatted emails are probably fine. But reaching outside Outlook's very specific HTML confines can cause broken graphics and strange formatting. Hit the link for the menu options and shortcuts that set Outlook 2003 and 2007 to send in "the telegram simplicity of raw text that ensures your message will be received clearly on any device".
Change from HTML to Plain Text in Outlook [ Wired How-To Wiki]