Molly explains what’s up with CSS and Outlook 2007
From the post:
As it turns out, in past versions, Outlook used two rendering engines. IE’s for reading content, and Word for composing messages. What this meant was that if you were replying or forwarding HTML emails, previous versions of Outlook would first use IE’s rendering engine to view it, then would switch over to the compose engine, Word.
Read the entire post for an explanation.
Related: Microsoft Outlook 2007: What you need to know when developing e-mail newsletters