Block failure notice emails in qmail

I have Plesk on my Linux server and I receive lots of spam emails from the root account on my server.
Here’s what happened. My forum used to send emails to non-existent email accounts around the web and I used to receive the failure notices for them.
To block failure notice emails in qmail that were forwarded to my main email I had to modify the aliases from

/var/qmail/alias

Here’s a sample email I received:

MAILER-DAEMON@nixware.net
Mar 30 (7 days ago)

to postmaster 
Hi. This is the qmail-send program at nixware.net.
I tried to deliver a bounce message to this address, but the bounce bounced!

:
2a00:1450:4013:0c01:0000:0000:0000:001a does not like recipient.
Remote host said: 550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at
550 5.1.1 http://support.google.com/mail/bin/answer.py?answer=6596 q5si14669450wjx.9 - gsmtp
Giving up on 2a00:1450:4013:0c01:0000:0000:0000:001a.

--- Below this line is the original bounce.

Return-Path: <>
Received: (qmail 8634 invoked for bounce); 29 Mar 2015 23:50:02 +0200
Date: 29 Mar 2015 23:50:02 +0200
From: MAILER-DAEMON@nixware.net
To: root@nixware.net
Subject: failure notice

As you can see, the postmaster account was receiving the spam from MAILER-DAEMON and MAILER-DAEMON was forwarding them to my gmail.com account. Pretty nasty…

The fix:
Go to

/var/qmail/alias

do a

ls -alh

and you will see a couple of hidden config files.

Inside each of those file I had my @gmail account. With the next script I overwrote the gmail account with a non-existent email:

for FILE in ./.qmail-*;do echo "nonexistent@nodomain.tld" > $FILE;done

Restart/reload qmail:

service qmail reload

That’s it! You should not receive any spam from any of the root, mailman or postmaster accounts.