I see spam going through my SMTP server, how can I prevent it

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If you look at your log file and see a lot of spam messages going through, or just some messages you do not recognize, you may have opened your server up and made it an open relay.

Here is what you need to check in your settings:

1. Check "How users will be authenticated" in IP Range List panel like on the picture. If you have it set to "Authentication is not required. Anyone will be able to relay", you open up your server completely and it will become an open relay. Spammers will find it out very quickly and start sending spam through it.
2. Check that you have enabled "Filter connections by IP address". If it is set to "Do not filter at all", it will have the same effect as above, so make sure it is set to either "Allow connections from IPs below" or "Deny connections from IPs below". Do not ever disable this filtering, unless you are in a closed Intranet environment, or you have a separate firewall filtering your connections before the server.
3. If you configure internal IP ranges for your router, make sure you exclude your router from the list of IP addresses, otherwise all connections coming from the router (which means from the outside world) will also be allowed. You only want to allow people on the internal network, not on the whole Internet.