Some PC users complain that mail messages forwarded from Apple mail come out blank, with the entire message as an attachment. This can be caused by either the Microsoft mail server the person is using, or a bug in Outlook Express on their Windows PC.

Microsoft write about the problem and how to fix it on the PC here:
http://support2.microsoft.com/default.aspx?scid=kb;en-us;312351&Product=oex

Microsoft write about how to fix it on the actual Exchange Server here:

http://support.microsoft.com/kb/969854

Rather than fix the bug from the PC end, which you probably do not have control over, there are some things that you can do from the Macintosh side.

Try this:

Go to Mail > Preferences > Composing, and set “Message Format” to plain text.
Go to Edit > Attachments and check ‘Always Send Windows-Friendly Attachments’

If you are desperate, this app claims to fix it from the Macintosh side for $15:
http://lokiware.info/Attachment-Tamer

More information here:
http://kb.mit.edu/confluence/pages/viewpage.action?pageId=4981187