Test for open relay
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How can I test if my firewall is being used for an email relay? I have landed on a couple of block lists, with the IP for the firewall being listed as the culprit. There is a mail server behind the firewall, and I have tested that and confirmed it is not relaying. That only leaves the firewall to be checked.
Stephen Keating
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Unless you installed postfix, it's not going to be the firewall. Probably some kind of spam mailer on a workstation. Add a rule to allow outbound smtp from your mail server, then block and log all other smtp traffic from the lan.
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Thanks
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MXToolbox has a bunch of tools, including an open relay test during its diagnostics:
http://mxtoolbox.com/
http://mxtoolbox.com/diagnostic.aspx