A Sender Policy Framework (SPF) record is a DNS (Domain Name System) record that helps prevent email spoofing and unauthorized use of your domain in email messages. It is a text record added to your domain’s DNS settings that specifies which mail servers are authorized to send email on behalf of your domain.

When an email server receives a message claiming to be from your domain, it can check the SPF record to verify if the sending server is authorized to send email for that domain. The SPF record lists the IP addresses or domain names of the authorized mail servers. If the sending server’s IP address matches one of the authorized entries in the SPF record, the email is considered legitimate. If the sending server is not listed or does not match the authorized entries, the email may be flagged as suspicious or even rejected.

The SPF record uses a specific syntax to define the authorized servers, such as including an “a” or “mx” mechanism to specify the domain’s own servers or its mail exchange servers, respectively. It also allows for the use of “include” mechanisms to include other domains’ SPF records, and “all” mechanism to indicate the default action for servers that are not explicitly mentioned.

By setting up an SPF record for your domain, you can enhance the authenticity and deliverability of your outgoing email messages while protecting your domain from unauthorized use in spoofed emails. It helps prevent phishing attempts, spam, and email-based attacks that rely on impersonating your domain.

Why PTR shouldn’t be used in an SPF Record:

Performance Issues: Using PTR records in SPF records can lead to additional DNS queries, which can slow down the email validation process. Since email systems need to respond quickly, this additional delay can be problematic.

Unreliability: The use of PTR records for validation is considered unreliable because reverse DNS setups might not be properly maintained by all domain owners, leading to inconsistencies and potential false positives in SPF checks.

Not Recommended Practice: According to the SPF specification (RFC 7208), the use of PTR records in SPF has been discouraged. Section 5.5 of RFC 7208 explicitly states that “PTR mechanisms SHOULD NOT be used.” The rationale is that PTR records are not a reliable method for verifying sender identity, which is the primary purpose of SPF records.