I found our mail service node is happily accepting spam sent to root@anyuserdomain and storing it in /var/mail/root, which will of course never be checked and grow forever. What is the best way to turn that off to block the spam? I would prefer to block invalid addresses rather than, say, link the alias to /dev/null and accept and delete the spam. Our pre-PPA hosting system (using Sendmail) inserted an @example.com virtmap for all user domains so mail would not be processed by aliases or user IDs. Ideally, we would still be able to have the "root" alias forward to an external email address for say crontab or other output, though for crontab specifically, we can of course add a MAILTO directive.