*This tutorial applies to Red Hat servers using Qmail through Xinetd*
Sometimes spammers will try sending hundreds, maybe thousands, of e-mails to your domain account using a random username each time in the hopes that eventually one will work. Unfortunately, these e-mails have bogus return addresses, so the sender and recipient bounce them back and forth forever, creating endless lag on your server and a hefty queue to clear out every day.
In this case you should create a "doublebounceto" file.
Before you do this, however, make sure you stop qmail and xinetd:
service qmail stop
service xinetd stop
Then check the qmail queue to make sure that no additional mail will be sent and subsequently bounced back. I use qmHandle for this (if you don't have qmHandle, GET IT):
/var/qmail/bin/qmHandle -l (This will show you the local queue)
/var/qmail/bin/qmHandle -D (This will delete everything in your queue)
Now navigate to the /var/qmail/control/ directory and add the doublebounceto file:
Add this and save:
| > /dev/null
Now start qmail and xinetd again:
service qmail start
service xinetd start
From netstat (netstat -nalp), you can see that they are still sending those e-mails, but they're basically just hitting the server and bouncing into a blackhole.
(As a precaution, I also added the | > /dev/null string to the .qmail-default files in /var/qmail/mailnames/domainname/)
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