By default, SLmail allows any server to use your SMTP server for SMTP relaying. That is to say, any Internet user who knows the name or IP address of your mail server can freely route all of their email through SLmail, potentially using large amounts of your system resources without your consent. The Relay Filtering page of the Web-based administration utility allows you to restrict who is allowed to relay mail through your server.
SLmail considers mail to be relaying if it is addressed to a domain which is not local to itself. SLmail considers the following areas when deciding whether or not a domain is local: