Using SORBS
www.sorbs.net › general › usingsmtp.dnsbl.sorbs.net - List of Open SMTP relay servers. web.dnsbl.sorbs.net - List of web (WWW) servers which have spammer abusable vulnerabilities (e.g. FormMail scripts) Note: This zone now includes non-webserver IP addresses that have abusable vulnerabilities.
SMTP - SMTP Open Relay
https://mxtoolbox.com/problem/smtp/smtp-open-relay?hide...More Information About Smtp Open Relay. During our diagnostics we attempt to simulate sending a message to a fake email address; test@example.com. We do this to try to detect if your server is an open relay, which means that it accepts mail to domains for which it is not responsible and then passes it along to the proper server.