Outlook 2007/2010/2013 utilizes the Word 2007/2010/2013 rendering engine. As a result, several CSS properties are disabled such as the background image or certain HTML elements (form, button, script).
This is one of the reasons we recommend including a link to the web version of your email. Recipients who click that link will be able to see your message properly in their browser.
Mailify allows you to host your web version in the Cloud regardless of your account level.