Just mention the term ‘DNS’, and many email senders’ eyes glaze over; say “reverse DNS” or “rDNS” and a look of panic may replace the glaze. Yet, not only are these not complicated concepts, but having reverse DNS set up is crucial to having consistent, good email delivery and deliverability. So it’s important to have a good understanding of what DNS and reverse DNS are, and do.

Just What is DNS, Reverse DNS, and rDNS?

DNS, which stands for Domain Name System, translates domain names to the actual number-based address (known as an ‘IP address’) on the Internet at which the computer for that domain resides, allowing computers to find each other, such as to visit a website. Or, to send email to someone whose email is hosted at that computer.

Remember Lily Tomlin’s switchboard operator, Ernestine? Or, the Hooterville switchboard in Green Acres? (Are we dating ourselves here?) When Oliver Douglas wanted to talk with Sam Drucker, he picked up the phone, and told the operator “Connect me to Sam Drucker”, and the operator translated that to “connect Oliver to Sam’s telephone number.”

You can think of DNS as like those operators – you tell your computer “send email to thatdomain.com” and the DNS system translates that to thatdomain.com’s IP address.

So, for example, when you send email to us at our address at isipp.com, your computer actually checks the DNS for isipp.com – and it finds out that the server which you think of as “isipp.com” is really known on the Internet as “that computer at IP address”

That IP address is what your computer needs to actually connect to our computer, and to transmit the email – to actually transfer the email from your computer to ours.

rDNS, or “reverse DNS”, allows the process to go the other way – when your computer connects to our computer to deliver an email message, it allows our computer to look at your computer’s IP address, and see what domain name your IP address claims to be.

Where DNS is like that switchboard operator, rDNS is like caller i.d.. Just as when you call us, our caller i.d. system takes your telephone number, and translates it to your name to be displayed on our caller i.d. box, so the rDNS system takes your computer’s IP address, and translates it to your computer’s domain name.

Here, roughly, is how that email transmission transaction works, in plain English. Let’s say you are trying to send email to us, and you are sending it from “yourplace.com”.

Your computer – the one which sends your email from “yo*@yo*******.com” – connects to our computer, and says “hello, I’m the computer at yourplace.com, and I have email for Support at Support’s email address there.”

Our computer says “Ok, I have that email address for Support here, so I can accept and deliver email to them from you.”

“But FIRST,” our computer goes on to say (particularly in this day and age of spamming, spoofing, and phishing), “..first…let me check to make sure that you are really who you claim to be.”

Then our computer takes note of the IP address from which your computer – the computer claiming to be “yourplace.com” – is connecting to our computer. Let’s say that IP address is

Our computer now has two pieces of information about you and your computer – your IP address, and that you are claiming to be “yourplace.com”.

If you are legitimate, then when our computer does a reverse DNS (rDNS) lookup, and asks the DNS system “what is the domain of the computer located at”, the answer needs to be “that computer is more commonly known as “yourplace.com”.

Then our computer knows that you are really who you say you are.

And delivers your email to us.

But if the DNS system says “that computer is more commonly known as bigbadspammer.com”, well, our computer is going to refuse to accept email from you.

That probably makes sense to you. Perhaps you even knew that.

But what you may not have known is that if the DNS system says “that computer is more commonly known as..known as…well…we have no idea what it’s more commonly known as because they don’t have rDNS set up”, well, then a lot of computer systems, ISPs, and inbox providers (including some big inbox providers) will refuse to deliver your email because they don’t know if they can trust you.

At the very least, even if they don’t outright refuse your email, they will immediately and seriously downgrade the trust they give it, and just one more problem (such as a moderate number of spam complaints for email coming from your IP address due to poor opt-in quality, or poor scheduling, or having repurposed transactional email addresses) can tip the scales so that your email gets rejected.

And many, if not most, major ISPs and inbox providers now require working rDNS before they will even think about whitelisting you.

To determine whether your IP addresses have rDNS set up, you can use our handy tool to test whether you have rDNS set up.

If you don’t have rDNS set up, it’s not hard to get rDNS set up for your IP addresses. Usually it just takes a phone call or email to the support department of your ISP. If you are using an ESP (email service provider) to send out your email, you’ll want to talk to them.

The Importance of Reverse DNS (rDNS) for Good Email Deliverability

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