Detecting SMTP open relays

Open relays are insecure mail servers that allow third-party domains to use them without authorization. They are abused by spammers and phishers, and they present a serious risk to organizations because public spam blacklists may add the relay servers and affect the entire organization depending on e-mails reaching its destination.

This recipe shows how to detect SMTP open relays with Nmap.

How to do it...

Open your terminal and enter the following Nmap command:

$ nmap -sV --script smtp-open-relay -v <target>  

The output returns the number of tests that passed, if the SMTP server is an open relay, and the command combination used:

   Host script results:
   | smtp-open-relay: Server is an open relay (1/16 tests)
   |_MAIL FROM:&lt...