Is it just this user that is seeing the issue or is it when anyone forwards a message using their blackberry the attachment is stripped?
Also, are the attachments from a particular sender or from any sender? Are they on Outlook or a different client?
I've seen an issue similar to this when a blackberry user was on Groupwise but the sender was using Exchange. Somewhere during the process, the attachment had gotten stripped from the e-mail.
Also, does the blackberry receive the attachment? Because a forward is just a call to the server to forward the message so really shouldn't have much to do with the blackberry, if that makes sense..