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The Importance of a PTR Record

SMTP, or Simple Mail Transfer Protocol, is a protocol used for sending and relaying messages over networks. SMTP defines rules and conventions for how email messages should be transferred and submitted.

When implementing an in-house-SMTP server, some requirements are needed to be fulfilled, including:

  • Domain Name
  • MX and A Records
  • Static IP address
  • Allow port 25 in Firewall
Once these requirements are met, you can receive emails. However, in terms of sending out emails to other servers, you will need an additional record, called the PTR record.

What is a PTR Record?

A PTR record, or a reverse DNS record, is a type of record that is used for validation. While a regular DNS record resolves a domain name to an IP address, PTR does the opposite. It resolves an IP address to a domain name. For example, if you have a MX record, myemail.mydomain.com, that points to 11.12.13.14, then a PTR record has the value 11.12.13.14 that points to myemail.mydomain.com

Why is a PTR Record Important?

The PTR record is important for the following reasons:

  • Authentication and Verfication: PTR records help SMTP servers verify that the IP address of the sending server connects to a legitimate domain name. This helps prevent email spoofing.
  • Spam Prevention: Many SMTP servers use PTR records to calculate the likelihood that an incoming email is spam. A lack of a PTR record or a mismatch between the domain and IP address can raise suspicion and will result in the email being flagged as spam.
  • Reputation Checking: SMTP servers often have databases to track the behavior of sending servers. PTR records affect the reputation of an email server. A correct PTR record can positively impact the sender's reputation, increasing the likelihood of successful email delivery.

How do I add a PTR record?

A PTR record goes into an ISP's DNS server. Therefore, you will need to consult with your ISP to add this record, since they are the one who owns the IP Address. For example, if you get an IP address from Verizon FIOS, this record goes into Verizon's DNS server, not yours. Please note that you will need a static IP to get a PTR record since they will not add it if you have a dynamic address.

How do I verify my PTR record?

You can use the nslookup command to confirm the Domain name and PTR record match. Below is an example for Synametrics.com:
  • You run the following command from any computer to get the IP address for www.synametrics.com

    nslookup www.synametrics.com
  • This should return 52.168.18.8
  • You run the following command to get a hostname back from the IP:
    nslookup -q=ptr 52.168.18.8
    This should return www.synametrics.com back again.


Conclusion

PTR records play an important role in email delivery by providing a way of verifying the sending mail server. Proper configuration of PTR records is essential to prevent your emails from being marked as junk and to have a good reputation for other SMTP servers. When configuring an email server, it is essential to make sure your PTR record exists and has the correct value assigned to it. In addition to PTR, please see google's sender guidelines on following best practices.


Created on: Jan 8, 2024
Last updated on: Jun 24, 2024

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