How to Test for Open Mail Relays

This guest blog explains the risk of "partially open mail relays"—SMTP misconfigurations that allow relaying from external to internal addresses or between internal addresses—demonstrates how to test for them using telnet/netcat, port checks, dig/nslookup and a Metasploit module, and recommends mitigations including requiring authentication, using STARTTLS, and restricting allowed sender IPs.