PTR and Reverse DNS: Simple Guide for Email and DNS

Most people only think about DNS in one direction: turning a domain like example.com into an IP address. But there is another, less visible side called reverse DNS, which does the opposite. It starts with an IP address and looks up the matching hostname. This is where PTR records come in.

Reverse DNS and PTR records matter a lot for mail servers and some security checks. If you run your own mail server or have a dedicated IP address from a hosting provider, it is worth understanding what PTR records do and why they can affect whether your emails are trusted.

What Is Reverse DNS?

Reverse DNS (rDNS) is the process of finding the hostname associated with an IP address. Instead of asking “what is the IP of example.com?”, you ask “what hostname does this IP 198.51.100.10 belong to?”

In normal (forward) DNS, we use records like A and AAAA to map names to addresses. In reverse DNS, we use a different special zone and PTR records to map addresses back to names.

This is especially important for:

Mail servers checking if a connecting IP looks legitimate.
Network diagnostics and logging (showing hostnames instead of raw IPs).
Some security tools and spam filters that use rDNS as one of many signals.

What Is a PTR Record?

A PTR record (Pointer record) is the DNS record used in reverse DNS lookups. It connects an IP address to a hostname, like the opposite of an A or AAAA record.

In simple terms, a PTR record says:

“For this IP address, the canonical hostname is something.example.com.”

Mail servers often check PTR records for the IP of the connecting server. If the PTR is missing, looks random, or points to a name that does not make sense, some providers treat the connection with more suspicion.

How Reverse DNS Zones Are Structured

Reverse DNS uses special domains like in-addr.arpa (for IPv4) and ip6.arpa (for IPv6). The IP address is basically reversed and turned into a name inside these zones.

IPv4 Example

For the IPv4 address 198.51.100.10, the reverse DNS name is:

10.100.51.198.in-addr.arpa.

In that zone, you might see a PTR record like:

10.100.51.198.in-addr.arpa.    PTR    mail.example.com.

This means “the host name for IP 198.51.100.10 is mail.example.com.”

IPv6 Example

IPv6 reverse DNS uses the ip6.arpa zone and expands each nibble (hex digit). The syntax is more complex and rarely managed manually by small setups. In most cases, your provider handles IPv6 PTRs on request.

Who Controls PTR Records?

This part confuses many people: you usually cannot set PTR records in the same DNS panel where you manage your A, AAAA, MX, and TXT records.

PTR records are controlled by whoever manages the IP address block — typically:

Your hosting provider or VPS provider.
Your ISP (for office or home connections).
A data center or cloud provider.

So if you want to change the reverse DNS for your server’s IP, you usually have to:

Look for a “reverse DNS / rDNS” option in your hosting control panel, or
Open a support ticket and ask the provider to set the PTR record for you.

You cannot fix PTR by adding records in your normal domain zone; it has to be done at the network owner’s side.

Why Mail Servers Care About PTR and Reverse DNS

Mail servers use many signals to decide whether to accept, reject, or filter a message. Reverse DNS is one of those signals, especially when they see a direct SMTP connection from a public IP.

Common checks include:

Does the IP have a PTR record at all?
Does the PTR look like a proper hostname (for example, mail.example.com) or something generic like 123-45-67-89.isp.example.net?
Does the hostname in the PTR resolve back to the same IP via an A/AAAA record? (Forward-confirmed reverse DNS)

If the answers to these questions look good, your server appears more legitimate. If not, some providers may apply stricter spam filtering or even refuse the connection, especially for servers trying to deliver email directly.

Forward-Confirmed Reverse DNS (FCrDNS)

Many admins aim for forward-confirmed reverse DNS, which simply means forward and reverse lookups agree with each other.

Ideal pattern for an email server:

PTR (reverse) lookup:
```
198.51.100.10 → mail.example.com
```
Forward lookup:
```
mail.example.com → 198.51.100.10
```

When both directions match, it is easier for receiving servers to trust that this host really belongs to your domain and is not some random compromised machine.

Setting Up Reverse DNS for a Mail Server

If you run your own mail server or use a dedicated IP from a provider, it is good practice to align your PTR, hostname, and DNS records.

Step 1: Pick a Clear Hostname

Choose a hostname that fits your domain and use it consistently, for example:

mail.example.com
smtp.example.com

Set this name as the server’s hostname in your mail software and system configuration.

Step 2: Create an A/AAAA Record

In your domain’s DNS zone, point that hostname to your server’s IP:

mail.example.com.    A      198.51.100.10
mail.example.com.    AAAA   2001:db8:1234::10   ; if you have IPv6

Step 3: Ask Your Provider to Set the PTR

Now contact your hosting provider or use their control panel to set the PTR for 198.51.100.10 to mail.example.com. Some providers let you enter this yourself; others do it via support.

After that, a reverse lookup for 198.51.100.10 should return mail.example.com, and a forward lookup for mail.example.com should return the same IP.

Step 4: Test the Setup

Use DNS tools to check both directions:

Reverse: lookup PTR for your IP.
Forward: lookup A/AAAA for your mail hostname.

You can also send test emails to providers like Gmail or Outlook and inspect the headers to see how they describe your connecting host.

Reverse DNS for Shared vs Dedicated IPs

If you use shared hosting or a shared outgoing mail IP, you usually do not control reverse DNS yourself.

Shared IP – multiple customers send email from the same IP. The provider sets a generic PTR like mail.sharedhost.com. You rely on their reputation and configuration.
Dedicated IP – only your domain uses the IP for mail. You can often choose a custom PTR that matches your domain (for example, mail.example.com).

If email is critical for your business and you send a lot of it, having a dedicated IP with properly configured reverse DNS can be a big advantage.

Other Uses of PTR and Reverse DNS

Although email is the most common reason people hear about reverse DNS, PTR records are also used in other areas:

Logging tools that show hostnames instead of raw IPs for easier reading.
Some security systems that perform additional checks on connecting hosts.
Network troubleshooting, where knowing the hostname can reveal the owner of an IP block.

However, for most domain owners, email deliverability remains the main reason to pay attention to PTR records.

Common PTR and Reverse DNS Problems

When email delivery is unstable, missing or incorrect reverse DNS is often part of the story. Some typical issues include:

No PTR record at all – many mail providers distrust servers that connect from IPs with no reverse DNS.
PTR points to a generic ISP name – not always fatal, but looks less professional than a dedicated hostname.
PTR name does not resolve back to the same IP – breaks forward-confirmed reverse DNS and may reduce trust.
Multiple hostnames pointing at the same IP with confusing PTR – can make troubleshooting harder.

If you ever see rejection messages mentioning “PTR,” “reverse DNS,” or “no rDNS,” that is a strong hint that this area needs attention.

Key Takeaways About PTR and Reverse DNS

Reverse DNS maps IP addresses back to hostnames using PTR records.
PTR records live in special reverse zones managed by the owner of the IP range, not in your normal domain zone.
Many mail servers check reverse DNS as one signal of trust for incoming connections.
For your own mail server, aligning PTR, hostname, and A/AAAA records is a best practice.
If you do not control the IP (for example, on shared hosting), you usually cannot control the PTR, but you can still ask your provider about their configuration.

PTR and Reverse DNS FAQ

What is a PTR record in DNS?

A PTR record is a DNS entry used in reverse DNS lookups. It connects an IP address to a hostname, effectively answering the question “which name belongs to this IP?” It is the opposite of an A or AAAA record, which maps a name to an IP address.

Why is reverse DNS important for email?

Many mail servers check reverse DNS to see if the connecting IP belongs to a properly named host. If the IP has a meaningful PTR record and that name resolves back to the same IP, the server looks more trustworthy. Missing or inconsistent rDNS can contribute to spam filtering or rejections.

Can I set PTR records in my domain’s DNS panel?

Usually not. PTR records are controlled by the owner of the IP address block — often your hosting provider or ISP. You can create A, AAAA, MX, and TXT records in your own zone, but PTR changes must be done through the provider that owns the IP range.

Do I need reverse DNS if I do not run my own mail server?

If you send email through a hosted service or shared provider, they typically manage reverse DNS for you. You do not need to configure PTR records yourself. However, if you host your own mail server or have a dedicated IP, setting proper rDNS is strongly recommended.

How can I check the PTR record for my IP?

You can use online reverse DNS lookup tools or command-line utilities like dig -x <IP>. The result will show the hostname returned by the PTR record, if one exists. If there is no PTR or it looks wrong, contact your provider to correct it.

PTR and Reverse DNS: Why Some Mail Servers Care About Your IP’s Name