Akbar,

SpamFilter is designed to handle incoming emails only. Relaying to domains not listed in your "local domains" is not allowed. If you need to use SpamFilter as an "outgoing smtp server" to relay emails to the internet, the alternative is to add the client's IP address(es) in an IP whitelist (whitelisting a domain won't work, you must use IPs since they cannot be faked by spammers...). In your case, if we understood your configuration correctly, you should add the IP address of your cable connection.