Connect to ssh ,and go to /etc/postfix/ dir.
Edit smtpd_recipient_restrictions line with the following line in main.cf :
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination,reject_non_fqdn_sender,reject_non_fqdn_recipient,reject_unknown_recipient_domain,reject_rbl_client zen.spamhaus.org,reject_rbl_client bl.spamcop.net,reject_rbl_client
Should "reject_rbl_client zen.spamhaus.org" not "reject_rbl_clientzen.spamhaus.org"