Using Postfix+Dovecot+MySQL, enable a catch-all+forward for a domain, while still allowing authenticated outgoing access

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http://serverfault.com – I have a server with Debian 6 (Squeeze), running Postfix, Dovecot, and using MySQL as the backing store for virtual email accounts. For my domain example.com, I have a ‘catch-all’ @example.com that forwards to [email protected] I want to allow an authenticated user to log in as [email protected] to use SMTP (and thus take advantage of SPF and DKIM validation), but not allow Postfix to deliver mail sent to [email protected] to Dovecot. Configuring a ‘full’ account in the MySQL database causes this to happen, so far as I can tell. How can I do this? Am I approaching this in the wrong way (HowTos)