With GBHackers Email spoofing Tool you can test that your server configured with an open relay. An open relay is an SMTP server configured in such a way that allows a third party to relay (send/receive email messages that are neither from nor for local users) Such servers are usually targeted by spam senders to send spoofed emails to victims inbox. If the mail server is vulnerable it shows Your message is on the way 147, if it is not vulnerable then it throws an error message.”]
Source: https://gbhackers.com/email-spoofing-tool-open-relay/