email authentication

Why Should You Avoid SPF PTR?

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It is only prudent to avoid using PTR for SPF email authentication. Considering the inconveniences and errors it causes, one must adopt one of the several other better alternatives.

PTR, which used to be integral to SPF, is no longer significant to email authentication. Its importance has waned over the years, and instead of adding value, it now introduces errors, delays, and inconveniences. Below is a detailed look into PTR, what problems you could face while using it, and what alternatives to choose to overcome them. The article also points to more robust options like DMARC implementation.

What Is PTR in an SPF Record? 

PTR is one of the mechanisms used in SPF to help email authentication. Though there are various mechanisms used with SPF, the working of PTR is quite different from other tools that help email authentication.

The importance of PTR has diminished over time due to other advanced and user-friendly email authentication concepts like deploying DKIM and SPF together as in DMARC and its difficulty staying compatible with the various digital tools in use today.

What are the Drawbacks of Using PTR in Email Authentication?

PTR has been a longstanding mechanism in SPF records for email authentication. However, it is important to recognize the drawbacks associated with its usage. One major concern is the increased complexity and potential errors introduced by PTR. The reverse mapping technique used by PTR involves additional lookups, which can slow down the email authentication process and lead to delays in email delivery.

Moreover, PTR places a heavier load on the .arpa name servers due to the increased number of queries. This can impact the overall performance and reliability of the email authentication system. Additionally, large email receivers often skip the PTR mechanism due to caching limitations, which can negatively affect the reputation of the sending domain and potentially result in SPF validation errors.

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How Does the PTR Mechanism Function?

The PTR mechanism in an SPF record works in reverse compared to the ‘A’ mechanism. While the ‘A’ mechanism looks at the domain to make out the associated IP address, the PTR mechanism is concerned with the sender’s IP address. The IP address is converted to ‘’ and ‘’ formats for IPv4 and IPv6, respectively, using a reverse mapping technique to find out the associated domains.

Subsequently, a lookup is performed to find the IP address associated with the obtained domain. This IP address is then compared with the IP address of the existing domain. If they match, the domain is considered authentic, which eventually helps in email delivery.

Why You Should Not Use PTR  

PTR has already been deprecated, and RFC4408 discourages it due to certain inconveniences it causes. Owing to the additional lookups involved compared to the ‘A’ mechanism, PTR could slow down the email authentication process, cause errors, and increase the load on the .arpa name servers.

SPF validation error can occur with PTR as large receivers could skip the PTR mechanism altogether due to limitations in caching, which will affect email reputation significantly.

What Are Some PTR Alternatives?

SPF provides other mechanisms to use in place of PTR, such as ‘A,’ ‘MX,’ ‘IPv4,’ ‘IPv6,’ and ‘include.’

  • ‘A’: It helps verify whether the incoming IP address matches the one associated with the domain.
  • ‘MX’: It checks if the sending server’s IP address matches that in the MX records.
  • ‘IPv4’ and ‘IPv6’: These confirm that the IP address being authenticated matches the IPv4 address or IPv6 address, respectively.
  • ‘Include’: With this mechanism, you can include the SPF records from different domains to manage them centrally.

For efficient email authentication, you may also use a DMARC policy that combines SPF and DKIM strengths to authenticate emails. Besides, you get reports with details of every email. This DMARC analyzer tool gives insight into the entire email traffic and helps ensure email security.

Exploring Better Alternatives to PTR in SPF Records

Thankfully, SPF provides several alternative mechanisms that can be used in place of PTR to achieve reliable email authentication. The ‘A,’ ‘MX,’ ‘IPv4,’ ‘IPv6,’ and ‘include’ mechanisms offer viable options to authenticate email sources without the inconveniences associated with PTR.

The ‘A’ mechanism verifies if the incoming IP address matches the one associated with the domain, ensuring accurate authentication. Similarly, the ‘MX’ mechanism checks if the sending server’s IP address aligns with the MX records, providing a reliable validation method. The ‘IPv4’ and ‘IPv6’ mechanisms confirm that the IP address being authenticated matches the respective IPv4 or IPv6 address. Lastly, the ‘include’ mechanism allows the central management of SPF records from different domains, streamlining the authentication process.

Final Words: Why it Makes Sense to Use SPF

As evident from the above discussion, a sensible suggestion would be to avoid using PTR and adopt alternative mechanisms offered by SPF. Considering the problems and inconveniences caused by PTR and the availability of ample alternatives, it is only prudent to do so. Additional options like DKIM and DMARC may also be considered for a more robust email authentication process.

However, one must not implement and forget tools like DMARC. It is essential to monitor DMARC reports regularly to get a complete and comprehensive picture of the incoming and outgoing email traffic.

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