I have seen these error messages, in the 8.5.1 (and FP1) client, which display "Cannot save bitmap to disk" when sending or saving some emails.
Technotes state that this has something to do with an active OLE object. The only workaround, that I have found, is to use the Internet-Style forwarding or replying.
Hopefully this will be fixed in FP2 or 8.5.2.