here a little Explanation on how to set the Postfix MailServer as an Open Relay SMTP Server.
Just edit the File „etc/postfix/main.cf“ and change the following line:
mynetworks = 0.0.0.0/0
then restart the Postfix MailServer – like:
sudo service postfix restart
Now your Mail Server is an Open Relay.
You should only do this for Test Purposes. So It would be a major security Risk!