I am still stumped on this issue. I turned on verbose messages in master.cf and still cant find why it comes up with invalid user. According to the logs, it does correctly associate the forwarding address with the original recipient.
As near as I can tell, it may be trying to send to a local mailbox with the forwarding address, which of course won't be found, instead of sending it out to the mailhost of the forwarding address domain.
Any help would be appreciated - Thanks!