If you see this type of rejected email message:
host mxi5p.craigslist.org [126.96.36.199]: 554 5.7.1<unknown[192.168.123.231]>:Client host
it means there is a problem with the DNS setup of the computer that is trying to send email to craigslist.org email addresses. One way to (almost always) avoid the entire DNS configuration issue is to have your email program relay/send your email through the mail server of your ISP.
Feel free to point them to this page, and be sure to have a copy of the exact error/reject message for them.
Unfortunately, if you are receiving this error/reject message, there is nothing we at Craigslist can do to fix it. Feel free to post a message in our help desk forum for help from other Craigslist users.
For those wanting to understand what is happening "under the hood"...
NOTE! Fixing DNS issues can be complicated and is not something the average computer user should have to deal with... See note above about talking to your ISP! Anyways, here we go!
DNS is short for Domain Name System. It's the way that IP addresses get converted to hostnames, and hostnames get converted to IP addresses. It's used all over the Internet.
When a mail server attempts to send email to Craigslist.org email addresses, in order to prevent as much spam as possible, we try to be sure that the server is who it says it is. Validating that DNS is setup properly for the connecting server is a great way to stop a lot of spam because spammers tend to employ all sorts of tricks and non-standard/broken configurations in order to hide their identity.
One of the first things that happens when a computer attempts to deliver email to Craigslist.org is that our server looks at the IP address of that computer and tries to convert that IP address to a hostname, by doing what is called a Reverse DNS Lookup. A DNS tool called "host" is being used to provide the following examples.
pluto:~ user$ host 192.168.123.231
192.168.123.231.in-addr.arpa domain name pointer samplehost.example.com.
The next thing that happens is that we try to convert that hostname (samplehost.example.com) back into an IP address to see if it matches (192.168.123.231):
pluto:~ user$ host samplehost.example.com
samplehost.example.com has address 192.168.123.231
When the reverse and forward lookups match up properly, the connecting computer has been validated to continue to talk to our mail server. However, when they don't match up, or any part of that validation fails, you will get the rdns/dns failure message.
There can be several things that will cause failures:
The solution is to make sure your rDNS and DNS are setup properly. Explaining how to do that is beyond that scope of this document, but more information can be found at: