Cannot modify item when use Word to send attachment.

I'm writing a Outlook Add-in such that when an item is sent, it will append
additional text to the subject field and attach custom signature depends on
the subject.

Public Sub oOL_ItemSend(ByVal Item As Object, Cancel As Boolean)
Everything works fine expect when the mail is created by using the "Send to
(as attachemnt)" function from Word. The ItemSend event still run and I've
confirmed that the subject and the content of the mail has been changed
within the procedure. But when user received the mail, all modification
were gone.

The platform is:
Windows 2000 SP3
Outlook 2000 SR-1
Word 2000 SR-1

Can anyone help me on this?

Thanks,
Thomson Mui