Usually, when I see problems with sending or forwarding email it has something to do with the user's location document. I'd suggest checking the path and location of the user's ID file and mail file in the location doc. If these don't match the actual location of the files it might cause this error. It could also be that the location doc itself is corrupt, or the user's personal address book has a problem. You could try creating a new location doc, or you could refresh/replace the design of the user's address book if the location doc looks good.
DDickey