Who ever is your ISP needs to set that up. They are normally in charge of the DNS that would give your IP address a host name. So they can set it up. Here is more detail on the issue for you to give them: Preparation: The reverse DNS entry for an IP is found by reversing the IP, adding it to "in-addr.arpa", and looking up the PTR record. So, the reverse DNS entry for 188.8.131.52 is found by looking up the PTR record for 184.108.40.206.in-addr.arpa. All DNS requests start by asking the root servers, and they let us know what to do next. See How Reverse DNS Lookups Work for more information. How I am searching: Asking e.root-servers.net for 220.127.116.11.in-addr.arpa PTR record: e.root-servers.net says to go to figwort.arin.net. (zone: 208.in-addr.arpa.) Asking figwort.arin.net. for 18.104.22.168.in-addr.arpa PTR record: figwort.arin.net [22.214.171.124] says to go to NS2.SOFTLAYER.COM. (zone: 58.101.208.in-addr.arpa.) Asking NS2.SOFTLAYER.COM. for 126.96.36.199.in-addr.arpa PTR record: Reports 188.8.131.52-static.reverse.softlayer.com. [from 184.108.40.206] Answer: 220.127.116.11 PTR record: 18.104.22.168-static.reverse.softlayer.com. [TTL 86400s] [A=None] *ERROR* There is no A record for 22.214.171.124-static.reverse.softlayer.com. (may be negatively cached). -----Original message----- From: Brian Cummiskey brian at hondaswap.com Date: Wed, 01 Aug 2007 13:11:05 -0700 To: "thelist at lists.evolt.org" thelist at lists.evolt.org Subject: [thelist] mail server problems > Hi guys, > > this might be a little offtopic, but i'm having some issues with my mail > on my webserver.... > > > i can't seem to send to addresses that require reverse lookups, namely > craigslist. > > host mxi1s.craigslist.org [126.96.36.199]: 554 <unknown[188.8.131.52]>: > Client host rejected: rDNS/DNS validation failed. Please setup matching DNS and rDNS records: > > > how do i set up dns and matching rdns records? > > i have cpanel and am on RH enterprise if it matters.