Exim Prevent Mail Sent by Relay

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http://serverfault.com – Currently it appears that somehow someone is using my server to send spam as I'm seeing 100's of rejection emails from other email services (such as gmail) saying the message could not be delivered "return to sender" and sent to mydomain.com. Upon investigation these email are coming via RELAY from a remote IP and not actually from the server itself (e.g. a virus etc...) Is there anything I can do to prevent someone using my mail server (exim) for relay? I have LFD on my server that monitors emails sent by relay and it's sending me the below alert. I have LFD set to block the sender if mo (HowTos)