What happens if an SPF record includes itself?

If an SPF record includes itself, it creates an infinite recursive loop during the SPF lookup process. This quickly exhausts the 10-DNS-lookup limit, causing the SPF check to fail (result in a PermError or TempError). Consequently, legitimate emails from your domain are likely to be marked as spam or rejected by receiving mail servers.

Why is there a 10-DNS-lookup limit for SPF records?

The 10-DNS-lookup limit is a security and performance measure specified in RFC 7208. It prevents malicious actors from creating complex SPF records that could lead to denial-of-service attacks by forcing receiving mail servers to perform an excessive number of DNS queries. It also helps to ensure efficient email processing.

How can I check if my SPF record has a self-referencing include or other errors?

You can use an online SPF checker or DMARC monitoring platform like Suped . These tools will analyze your SPF record for syntax errors, lookup limit breaches, and other misconfigurations that could impact deliverability. DMARC reports also provide visibility into SPF authentication results.

What is SPF flattening and how does it help with the 10-DNS-lookup limit?

SPF flattening is a technique used to reduce the number of DNS lookups in an SPF record. It involves replacing mechanisms with the actual (IPv4 or IPv6) addresses of the included sending sources. This allows your SPF record to contain more authorized senders without exceeding the 10-lookup limit. Platforms like Suped offer SPF flattening as a feature.

Should all included SPF records have an '-all' mechanism?

No, typically only the primary SPF record for your domain should have a final mechanism. Included SPF records from third-party senders often use or no final mechanism at all, as their purpose is to contribute to your domain's overall policy, not to define a complete policy on their own. The main record's applies to the entire SPF evaluation.

Can an SPF record have an 'include' mechanism that points to itself? - SPF - Email authentication - Knowledge base

An email icon with a magnifying glass examining a complex chain of linked DNS records, illustrating SPF record lookups.

The world of email authentication can be complex, and SPF (Sender Policy Framework) is no exception. A common question that arises when configuring SPF records is whether an 'include' mechanism can point to the same domain where the SPF record is published. This might seem like a clever way to simplify complex setups, but it actually introduces a critical problem.

SPF records are designed to specify which mail servers are authorized to send email on behalf of a domain. The mechanism is particularly useful for delegating sending authority to third-party services, such as email service providers (ESPs) or transactional email platforms. It instructs receiving mail servers to check the SPF record of another domain to determine sender legitimacy.

However, using an mechanism to point to the very domain it resides on creates a recursive loop that violates fundamental SPF rules and can lead to significant email deliverability issues. Let's delve into why this configuration is problematic and what you should do instead.

Understanding the SPF 'include' mechanism

How the SPF 'include' mechanism works

The mechanism is a directive within an SPF record that tells a receiving mail server to evaluate another domain's SPF record as if it were part of the current domain's record. This is crucial for domains that rely on multiple external services to send email on their behalf, allowing them to consolidate their authorized sending sources efficiently. For example, an SPF record might include references to Google Workspace and an email marketing platform.

Example of a typical SPF record with multiple includesdns

v=spf1 include:_spf.google.com include:sendgrid.net -all

When a mail server receives an email, it performs an SPF check by looking up the sender's domain's SPF record. If an mechanism is found, the server then initiates a separate DNS query to retrieve the SPF record of the included domain. This process can be recursive, meaning an included domain's SPF record might itself contain another mechanism, leading to a chain of lookups. The mechanism is fundamental to modern email authentication, as detailed in this Cloudflare article on SPF records.

While multiple mechanisms are perfectly valid and common, as noted on Server Fault's discussion on SPF, the key is that these inclusions point to *different* domains or subdomains. The entire SPF lookup process is subject to a 10-DNS-lookup limit to prevent abuse and denial-of-service attacks. Each , , , , and mechanism, among others, typically counts as a DNS lookup. Understanding these limits is crucial for properly formatting SPF TXT records and avoiding DNS size issues, which we detail in our guide on formatting SPF TXT records.

The problem with self-referencing includes

When an SPF record contains an mechanism that points to the same domain where the record is published (e.g., ), it creates an infinite loop. The receiving mail server attempts to evaluate the current domain's SPF record, encounters the self-referencing , and then tries to evaluate the *same* record again. This recursive loop quickly exhausts the 10-DNS-lookup limit set by the SPF specification (RFC 7208).

A mail server icon with an infinite loop symbol around it, representing a recursive SPF lookup issue.

Once the 10-lookup limit is exceeded, the SPF check results in a or error, meaning the SPF authentication cannot be completed. This failure indicates that the sending server is not authorized, even if it actually is, leading to legitimate emails being marked as spam or rejected entirely. This is a common pitfall that can lead to an SPF TempError in your DMARC reports.

Invalid SPF 'include'

The SPF record includes itself, causing an infinite loop.

Example of a problematic SPF recorddns

v=spf1 include:yourdomain.com -all

The lookup process never resolves, leading to an immediate SPF failure.

Valid SPF 'include'

The SPF record includes a different domain's SPF record.

Example of a correctly configured SPF recorddns

v=spf1 include:thirdpartydomain.com -all

The lookup proceeds as expected, evaluating the included domain's SPF record.

Impact on email deliverability and security

A self-referencing SPF mechanism effectively breaks your SPF authentication. This has direct and negative consequences for your email deliverability, as receiving mail servers, especially those with strict policies like

Gmail and

Yahoo, will see a failed or softfail SPF result. This increases the likelihood that your legitimate emails will be sent to the spam folder, quarantined, or outright rejected. This issue often results in an SPF unauthorized mail is prohibited error.

Beyond deliverability, a broken SPF record also weakens your domain's email security posture. While SPF alone doesn't prevent all forms of spoofing, a correctly configured SPF record, especially when combined with DKIM and DMARC, helps to validate sender identity. A misconfigured SPF record can create ambiguity, making it harder for receiving servers to distinguish legitimate email from malicious attempts to impersonate your domain. This increases the risk of successful phishing attacks against your recipients or your brand's reputation.

The hidden SPF DNS timeout issue

Microsoft 365 can be particularly sensitive to SPF record issues, sometimes resulting in a hidden SPF DNS timeout. A self-referencing will exacerbate this, leading to more frequent deliverability failures for recipients using Microsoft's email services. Ensuring your SPF is correctly configured is essential.

Monitoring your DMARC reports is essential for identifying these types of SPF failures. Tools like Suped provide actionable insights, real-time alerts, and AI-powered recommendations to quickly diagnose and fix authentication issues, including recursive SPF problems. This helps maintain your domain's reputation and ensures your emails reach the inbox reliably.

Best practices for SPF records

To avoid issues with SPF records, particularly concerning the 10-DNS-lookup limit and recursive mechanisms, follow these best practices:

Avoid self-referencing includes: Never point an mechanism to the same domain (or a subdomain that resolves to the same SPF record) where it is published.
Consolidate your SPF record: Ensure your SPF record is a single TXT record for your domain. Having multiple 'v=spf1' declarations is another common mistake that can invalidate your SPF.
Use SPF flattening: If you have many mechanisms and are hitting the 10-DNS-lookup limit, consider SPF flattening. This technique replaces mechanisms with the actual IP addresses of the included domains, reducing the number of DNS lookups.
Regularly review your SPF record: As your sending infrastructure changes, so too should your SPF record. Periodically check that all included domains are still active and necessary, and that your record adheres to SPF best practices.

Implementing a robust DMARC monitoring solution like Suped can help automate much of this review process. Suped offers a unified platform for DMARC, SPF, and DKIM monitoring, alongside SPF flattening and real-time alerts. It provides AI-powered recommendations to streamline email authentication management for both SMBs and large enterprises, including Managed Service Providers.

Ensuring a robust SPF policy

While the idea of an SPF record including itself might seem intuitive for certain configurations, it’s a critical misstep that leads to immediate authentication failures. The SPF specification clearly defines the 10-DNS-lookup limit, which is quickly exhausted by any self-referencing mechanism.

Prioritizing correct SPF configuration is paramount for maintaining good email deliverability and protecting your domain from unauthorized use (spoofing). By adhering to best practices, consolidating your SPF record, and leveraging advanced tools for DMARC monitoring, you can ensure your emails are authenticated correctly and consistently reach their intended recipients.