# Example: Block a spammy ASN range 192.0.2.0/24 REJECT Spam from ASN 12345 203.0.113.0/24 REJECT Spam from ASN 67890
You will see something like: origin: AS15169 (Google’s ASN). Zimbra does not have a GUI for ASN filtering, but you can configure Postfix directly. asn zimbra
Review your zimbra.log for the specific rejection line. If you see Reason: ASN listed , you likely need to switch to a less aggressive RBL or whitelist a specific upstream relay. Disclaimer: ASN netblocks change frequently. Always verify CIDR ranges using a BGP looking glass before applying firewall rules. # Example: Block a spammy ASN range 192