HTML Emails
Here are some general resources on HTML emails:
A Guide to CSS Support in Email: 2007 Edition 
HTML Emails - Taming the Beast
How to Code HTML Email Newsletters
Here are some specific resources on HTML email rendering in Outlook 2007:
Microsoft Breaks HTML Email Rendering in Outlook 2007
Word 2007 HTML and CSS Rendering Capabilities in Outlook 2007 (Part 1 of 2)
Word 2007 HTML and CSS Rendering Capabilities in Outlook 2007 (Part 2 of 2)
