# Install a lookup tool (Ubuntu/Debian) apt install whois whois 8.8.8.8 | grep -i origin
Edit /opt/zimbra/common/conf/postfix_asn_blacklist Add the ASN netblocks (you must expand ASN to CIDR ranges manually or via script): asn zimbra
Since "ASN" can refer to several technical concepts (Autonomous System Number, Application Service Node, or an education provider like ASN), this article covers the most common and technically relevant scenarios for IT administrators and engineers. In the world of email server management, particularly for administrators handling Zimbra Collaboration Suite (ZCS) , logs and headers often present cryptic acronyms. One such term that frequently appears in mail logs, firewall rules, or anti-spam configurations is ASN . # Install a lookup tool (Ubuntu/Debian) apt install
postmap /opt/zimbra/common/conf/postfix_asn_blacklist Always verify CIDR ranges using a BGP looking
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.
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.