david writes "qmail is a mail transfer agent that runs on Unix. It was more secure replacement for the popular Sendmail program. The author offered a $500 prize for the first person to publish a verifiable security hole in the latest version of the software.
This is a useful thing to do in a number of situations. For instance, if you are hit with a spamming attack, you can temporarily instate a second Qmail installation (once the spam run is finished), allow it to take over mail receipt,and then use this tool to clean the offending mails out of the queue before switching over to the main Qmail installation once again."