Provide simpler postfix integration

Postfix would generally like to know whether schleuder is going to handle an incoming mail or not. If it is not going to handle it, it would be better to reject the mail at the time of smtp reciept, using smtpd_reject_unlisted_recipient.

This provides easier integration with postfix, as the comments in the file describe. You can add and remove new lists from schleuder, and postfix will know what addresses to reject automagically :)