This is postfix not being able to resolve the claimed domainnames that folks are using in their mail. What happens when you check the domain name of a failed sender address with host. eg for joe@xxxxxxxxxxx try "host exampleXXX.com". bash-3.1$ host exampleXXX.com Host exampleXXX.com not found: 3(NXDOMAIN) If your dns can't find even well-known domains (say redhat.com ;-)) then you'll want to fix that first. Postfix should then fix itself perhaps after a restart/reboot. I just run bind (aka named) locally. Using some isp's server usually creates problems like the above.
Thanks Wolfgang! Yes, it was a typo in my resolv.conf. So while a "dig domain.tld" found the MX and A records and an "nslookup domain.tld" found the NSs, a "host mit.edu" rendered nothing. And yeah, I've configured BIND to serve local (LAN) NS && DHCP services, while the resolver refers to external (WAN) NSs for external lookups. Thanks again :-) Stu@
