Use the Security Settings... | Relay Control menu selection to define how your server reacts to mail relaying. When a message arrives at your mail server that is neither from nor to a local address, your server is being asked to relay (or deliver) the message on behalf of another user. If you do not want your server to relay mail for unknown users you can use the settings provided.
Configuring MDaemon to be an open relay may cause your server to be listed on Real-Time Blacklists and some servers may refuse mail from you.This article refers to MDaemon 10.x and above. For information on how to resolve this in previous versions, please see the link below.