I am using Exim as my SMTP server and looking at the logs more carefully, it is Exim that errors with "550 - Unknown User".
Make sure you don't have check_local_user set in the Exim router that handles these mailboxes. See the Exim docs for details of that flag and side-effects of removing it.
You can test your config change with 'exim -bt [EMAIL PROTECTED]'.
-Pat
