What SPF mechanism checks for a valid pointer record?

When you're setting up your Sender Policy Framework (SPF) record, you'll come across several different "mechanisms". These are the specific instructions in your record that tell receiving mail servers how to check if an email is from an authorized source. One of the less common, and frankly, discouraged mechanisms is the one designed to check for a valid pointer record, or PTR record.

The specific SPF mechanism that checks for a valid PTR record is called the ptr mechanism. While it exists as part of the SPF specification, it's widely recommended that you avoid using it.

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The ptr mechanism is designed to perform a reverse DNS lookup to validate a sending server. A PTR record maps an IP address back to a domain name, which is the opposite of what a normal A record does (mapping a domain name to an IP address).

The validation process goes like this:

• A receiving mail server gets the IP address of the server trying to send an email.
• It performs a PTR lookup on that IP address to find the associated hostname(s).
• For each hostname returned, the server then performs a forward DNS lookup (for an A or AAAA record) to see what IP addresses are associated with that hostname.
• If any of the IP addresses from the forward lookup match the original sending IP address, and the hostname matches the domain specified in the ptr mechanism, the check passes.

Tools, Utilities and Calculators - WintelGuy.com says:

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The ptr mechanism checks for consistency between the sender's PTR and the A/AAAA records. Upon success it then compares the domain name obtained from the PTR...

Despite its seemingly logical approach, the ptr mechanism is heavily discouraged for several important reasons. In fact, the official RFC that defines SPF explicitly warns against its use.

IETF Datatracker says:

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This mechanism is slow, not as reliable as other mechanisms in cases of DNS errors, and places a large burden on the .arpa name servers. If used, proper PTR records have to be in place for the domain's hosts and the "ptr" mechanism SHOULD be one of the last mechanisms checked.

It's slow and inefficient. The multi-step lookup process (reverse, then forward) is resource-intensive. It requires multiple DNS queries for a single email, which can slow down email delivery and put an unnecessary load on DNS servers.

It's unreliable. Many valid sending servers do not have properly configured PTR records. Using this mechanism can lead to legitimate emails failing SPF checks and potentially being marked as spam or rejected entirely.

As experts point out, using it can be risky.

DuoCircle says:

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PTR: The SPF match is validated if the PTR record is linked to a given domain directed to the client's address. Experts discourage its use as it can block both ...

Instead of relying on the ptr mechanism, you should always use more reliable and standard mechanisms to build your SPF record. These directly specify which servers are allowed to send email for your domain.

The most common and recommended mechanisms are:

• a: Authorizes servers by the domain's A or AAAA records.
• mx: Authorizes the mail servers listed in the domain's MX records.
• ip4 / ip6: Authorizes specific IPv4 or IPv6 addresses or ranges.
• include: References the SPF record of another domain, typically used for third-party sending services like Google Workspace or Mailchimp.

By sticking to these standard mechanisms, you create a more efficient, reliable, and secure SPF record that accurately reflects your sending infrastructure without the risks associated with the ptr mechanism.