This was the error message we were getting while trying to email Craigslist.org email addresses and here is how to get some clues about this problem.

So login to your server and on the command line type:

> host IP_ADDRESS_OF_YOUR_SERVER

Note, the IP_ADDRESS_OF_YOUR_SERVER is in the same message from Craigslist.org just find the line ““; xxx.xxx.xxx.xxx will be an IP address.

Now when you run this you will get something like:

> BLAH_BLAH domain name pointer SOME_DOMAIN_NAME.

If you do the reverse and check for SOME_DOMAIN_NAME, you should get the same IP address as above:

> host SOME_DOMAIN_NAME
SOME_DOMAIN_NAME has address xxx.xxx.xxx.xxx

If the IP address doesn’t match the one you used above then your emails to Craigslist.org will bounce back with the message:

“Client host rejected: rDNS/DNS validation failed. Please setup matching DNS and rDNS”

Fix:

To fix this you have two options, using your ISPs mail server or fixing the issue above, if you choose to fix the issue above, then you must contact your hosting provider, because you most probably won’t have the option to do it directly from yous server, contact them and tell them that the result of these two host operations don’t match and they will fix it for you very quickly… Note: The person who can fix this is the person who is in charge of your network, so in most cases it’s your hosting provider or maybe your IP department (if the servers are in your office).