The return of the open relays

**Executive summary:** This report reviews the history and resurgence of email open-relay abuse used to send spam, explains two common modern misconfiguration scenarios (firewall/NAT rewriting inbound SMTP source IPs and misconfigured SMTP/security appliances that blindly forward mail), illustrates examples of offending headers, and recommends configuration and appliance fixes plus testing to prevent two-device open-relay mail systems.