It looks like one of nanog's outbound servers doesn't have a PTR record. Mark Received:from s0.nanog.org (207.75.116.162) by edge.atlasbiz.com (192.168.198.21 ) with Microsoft SMTP Server id 8.2.255.0; Sun, 13 Feb 2011 21:34:17 +0000 ; <<>> DiG 9.6.0-APPLE-P2 <<>> -x 207.75.116.162 ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 29686 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;162.116.75.207.in-addr.arpa. IN PTR ;; AUTHORITY SECTION: 116.75.207.in-addr.arpa. 10764 IN SOA dns.merit.net. ejd.merit.edu. 2011021202 28800 14400 2419200 14400 ;; Query time: 0 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Mon Feb 14 09:54:42 2011 ;; MSG SIZE rcvd: 107 -- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: marka@isc.org