"Outlook Express doesn't correctly handle MIME, and won't display the body of signed messages inline. Users get a blank email and two attachments (one of the message text and one of the signature) and therefore need to open an attachment to see the email."
http://en.wikipedia.org/wiki/Outlook_Express

For most this usually just works since the 8000 pound gorilla has forced some to send MIME that works with OLEXP. If it doesn't comply then there is no fix for this one.

Bob