If you are sending emails to business clients support for Outlook 2000-2003 & Outlook 2007 is still essential. I'm afraid you need to code them like it's 1999. MailChimp have a guide on how to do it right: http://mailchimp.com/resources/guides/email-jitsu/

Fortunately a <table> is (and always has been) responsive.

For non-business clients (who would be unlikely to use Outlook at all) these look great.