With the release of Office 2007 Microsoft has removed the Internet Explorer based rendering engine that Outlook used to use for email and replacing it with Microsoft Word.  Unfortunately this leaves Outlook 2007 with limited support for HTML and CSS that is built into Word 2007 to display HTML email messages.

Basically, unless your HTML emails are extremely simple, users of Outlook 2007 will have difficulty reading the message. 

Unfortunately the only solution to those problems will be to reduce the complexity of your HTML email design to accommodate Outlook’s limited feature set.  With the overwhelming majority of users using Outlook (versus other email clients), this cannot be ignored.

There are a number of limitations but here are the main ones:

•     no support for background images (HTML or CSS)  
•     no support for forms  
•     no support for Flash, or other plug ins  
•     no support for CSS floats  
•     no support for replacing bullets with images in unordered lists  
•     no support for CSS positioning  
•     no support for animated GIF's  

Many newsletter designs that have displayed perfectly until now may display incorrectly in Outlook 2007. We recommend testing all your designs in future by sending a test message to an Outlook 2007 email client. You may need to make design compromises in order to get your newsletters to display correctly.

Thunderbird and many other email clients still support most of these features, but since Microsoft Outlook has 75% of the market it is wise to gear your templates for the change immediately.