Cybercriminals using Email spoofing methods to deliver forged emails to recipients. the email servers that are available publically available can be used for Email spoofing attack. 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).Therefore, such servers are usually targeted by spam senders to send spoofed emails to victims inbox.
You can read our article on Email Spoofing – Exploiting Open Relay configured Public Mailservers for more details.