main.cf:
smtpd_recipient_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_non_fqdn_hostname,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unauth_destination,
reject_unauth_pipelining,
reject_invalid_hostname,
reject_rbl_client bl.spamcop.net,
reject_rbl_client cbl.abuseat.org,
reject_rbl_client dnsbl.sorbs.net,
reject_rbl_client zen.spamhaus.org
For zen.spamhaus.org you need to register an account and give them the IP and/or hostname for the mail server in use. If you don't do so, it will not allow you to use the service. I use zen.spamhaus.org and it works fine.