Reverse DNS

What is rDNS?

If you get pulled over by a cop, the first thing they ask for is your license and registration. If the name on your registration matches your driver’s license then the cop is able to verify that you are the car owner.

Reverse DNS works the same way. rDNS is a type of email authentication that is used to match your mail server IP to your host name.

How does rDNS work?

When you send an email, your recipient’s mail server will check to see if your Sending IP address matches the domain name within the HELO command. This is also known as HELO to IP.

HELO is a command sent by a mail server to begin the process of sending an email. It is used to identify your domain name with your Mail Server IP address.

Here’s a message header with HELO identifying the domain name of a mail server.

Why is rDNS important and how does it affect deliverability?

rDNS is important because it adds credibility to the mail server sending email and functions as an additional layer of email authentication. rDNS allows you to separate valid email senders versus compromised servers that are sending spam.

Another benefit of rDNS is that you don’t need to accept the entire message body to complete a rDNS lookup.

You only need information in your message header which is sent at the beginning of an email chain. If the email fails rDNS, you can refuse it which will save server resources and prevent spam from reaching your inbox.

Several major mailbox providers such as Gmail, Microsoft and Yahoo will block emails coming from a mail server without valid rDNS records. Also, some SMTP servers are configured to reject emails when the rDNS does not match the HELO.

However, keep in mind that mailbox providers place more weight on your overall IP address and domain reputation for deciding where your emails should be routed.

rDNS is only one variable of many that will determine where your message should be delivered.

What does MailGenius cover with regards to rDNS?

How do I setup rDNS?

If you use an ESP such as Google Apps or Office 365 they are responsible for configuring rDNS for all of their IP addresses. This is also the case if you’re using any email marketing platform like MailChimp to send emails.

You are only responsible for configuring rDNS if you are setting up a mail server, or your ESP has provided you with your own dedicated ip.

In order to setup rDNS you must meet the following requirements:
Step 1: Create a Reverse DNS Zone
  1. The hostname for the zone should start with a portion of your IP address written backwards followed by .in-addr.arpa.
  2. If your IP Address is 192.168.0.127, you need to remove the final octet which leaves you with 192.168.0
  3. Now you need to reverse 192.168.0 which is: 0.168.192
  4. Now append .in-addr.arpa to 192.168.0
  5. Now you have the completed reverse zone domain of 0.168.192.in-addr.arpa
Step 2: Create a PTR Record
  1. Add a new PTR record.
  2. Name it with the final digit of your IP address. In our example above it would be 127.
  3. For the Canonical Hostname, enter the domain name you’d like the IP address to resolve to: mailserver.example.com 

If you don’t meet the requirements above you may need to open a technical support ticket with your hosting provider or ESP and ask for the following:

Example rDNS records:

Additional Resources