smtpd_recipient_restrictions =
permit_mynetworks,
reject_unauth_destination
reject_unknown_sender_domain
check_recipient_access dbm:$config_directory/recipient_access,
check_client_access dbm:$config_directory/access,
check_client_access dbm:$config_directory/helo_access,
reject_non_fqdn_hostname,
reject_invalid_hostname,
reject_rbl_client list.dsbl.org,
reject_rbl_client relays.ordb.org,
reject_rbl_client bl.spamcop.net,
check_sender_access dbm:$config_directory/spammers
check_sender_access dbm:$config_directory/junk
check_sender_access dbm:$config_directory/freemail_access,
reject_unauth_pipelining

reject_unknown_sender_domain is not rejecting. can anyone assist in
this.
thanks