Does your site send out HTML-based emails? If so, then you should be aware of the Email Standards Project web site. This site provides a plethora of information on how to ensure your HTML-based emails are rendered properly on a maximum number of email clients.
The site provides a super-simple email client scorecard on the homepage (surprise - Outlook is rated "poor") as well as an acid test for email clients.
If you're designing HTML emails - regardless of if you're using Drupal - this site is a must read.