There can be a wide variety of reason why an E-mail may not be accepted by our servers. We will go over a few of the more common scenarios below and discuss some ways to help alleviate these problems.
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 an Elexio hosted 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 an Elexio hosted E-mail account, 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:
- Reverse DNS is not setup for your IP address
- Forward DNS is not setup for the hostname that is returned by the reverse DNS query
- Forward DNS is setup for the hostname that is returned by the reverse DNS query, but the IP address it returns does not match the IP address of the computer that is attempting to connect to our mail servers.
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: