<https://www.plesk.com/kb/support/how-to-verify-that-the-server-is-not-acting-as-an-open-relay-on-plesk/#webpage>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://www.plesk.com/kb/support/how-to-verify-that-the-server-is-not-acting-as-an-open-relay-on-plesk/>