If you still wanted another one I don't see zen.spamhaus.org in your first post.
You really should look at some other options too (
that wiki article), if the RBL's aren't up and you're fully depending on them, well you're out of luck.
Host checks:
reject_invalid_hostname
reject_non_fqdn_hostname
reject_non_fqdn_sender
DNS checks:
reject_unknown_client
reject_unknown_hostname
reject_unknown_sender_domain
-Be careful with the reject_unknown_client & reject_unknown_hostname DNS checks, as they can block more than you think sometimes...
You can also change the entry in /opt/zimbra/conf/zmmta.cf for smtpd_reject_unlisted_recipients to 'yes', save the file & then do a 'postfix reload'.