Nether.net Open Relay Blacklist: Any host that sends email to an invalid username @ puck.nether.net or @nether.net is tested to ensure that it is not an open relay. Due to the proliferation of spam due to open relays, we have found this to be a necessity. We perform the same tests that are performed if you were to telnet to mail-abuse.org from your mail server. You can check those tests by doing so. It does a reverse-connect to your IP and tests it for a various set of ways that one can send email that would turn out to pass through anti-relaying features.

More information about NETHERUNSURE can be found at their website: http://puck.nether.net/or/

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