How to add spam filters on zimbra server.

# su - zimbra

Check current setting.
$ zmprov gacf | grep zimbraMtaRestriction
Output:
zimbraMtaRestriction: reject_non_fqdn_sender

Currently reject_non_fqdn_sender is set.
Adding sbl.spamhaus.org

$ zmprov mcf zimbraMtaRestriction reject_non_fqdn_sender zimbraMtaRestriction "reject_rbl_client sbl.spamhaus.org"

Check the result:
$ zmprov gacf | grep zimbraMtaRestriction
zimbraMtaRestriction: reject_non_fqdn_sender
zimbraMtaRestriction: reject_rbl_client sbl.spamhaus.org

Other options

zmprov mcf 

zimbraMtaRestriction reject_invalid_hostname 

zimbraMtaRestriction reject_non_fqdn_hostname 

zimbraMtaRestriction reject_non_fqdn_sender 

zimbraMtaRestriction "reject_rbl_client dnsbl.njabl.org" 

zimbraMtaRestriction "reject_rbl_client cbl.abuseat.org" 

zimbraMtaRestriction "reject_rbl_client bl.spamcop.net" 

zimbraMtaRestriction "reject_rbl_client dnsbl.sorbs.net" 

zimbraMtaRestriction "reject_rbl_client sbl.spamhaus.org" 

zimbraMtaRestriction "reject_rbl_client relays.mail-abuse.org"

According to zimbra wiki