An open mail relay is a setting email servers use to allow anyone to send email through it.
How to test an IP address for an open relay: Use the search term "SMTP open relay test" to find a 3rd party tool to test for open relay.
How to report an open relay: Search for "how to report open relay" to find a 3rd party tool to open relays.
How to fix an open relay: If you need network or system security advice, you should hire a competent accredited security consultant. If you feel comfortable troubleshooting on your own, search for "securing systems against spam" or "how to secure your email system against third-party relay".