What exactly is the SPF DNS lookup limit?

The SPF DNS lookup limit restricts the number of DNS queries performed when a mail server evaluates your SPF record to a maximum of 10. This is to prevent resource exhaustion and potential denial-of-service attacks against DNS infrastructure. Exceeding it results in an SPF PermError .

What happens if I exceed the SPF lookup limit?

If you exceed the limit, your SPF record will fail authentication with a PermError . This means recipient mail servers cannot validate your sending sources. Consequently, legitimate emails from your domain are likely to be rejected, quarantined to spam folders, or experience severe email deliverability issues .

How does this affect my DMARC policy?

A failed SPF check due to a PermError will cause SPF alignment to fail in your DMARC reports. If your DMARC policy relies on SPF for authentication, this will lead to a DMARC failure. This can prevent your domain from being protected against spoofing and phishing attacks and may result in your DMARC policy instructing recipient servers to quarantine or reject legitimate emails.

What is SPF flattening and how does it help?

SPF flattening is a technique to reduce the number of DNS lookups required by your SPF record. Instead of including multiple domains via include mechanisms, which each trigger a lookup, an SPF flattening service resolves these domains into a list of IP addresses. These IP addresses are then placed directly into your SPF record, often as an ip4 or ip6 mechanism, without counting towards the 10-lookup limit. Suped offers SPF flattening as a core feature.

What is the impact of exceeding the SPF DNS lookup limit? - SPF - Email authentication - Knowledge base

An illustration of an email server overwhelmed by DNS requests.

Understanding the nuances of email authentication is vital for ensuring your messages reach the inbox. One common pitfall that can severely impact your email program is exceeding the SPF DNS lookup limit. This isn't just a technical detail; it's a critical factor that can directly affect whether your emails are delivered, marked as spam, or rejected outright.

The SPF (Sender Policy Framework) record specifies which servers are authorized to send email on behalf of your domain. To check if a sending server is legitimate, an email receiver performs DNS lookups based on the mechanisms defined in your SPF record. However, there's a strict limit to how many of these lookups can occur during a single SPF check.

If your SPF record is too complex and requires more than the allowed number of DNS queries, it can lead to validation failures, even if all your listed sending sources are legitimate. This article will explore the consequences of hitting this limit and provide insights into mitigating its effects on your email deliverability and sender reputation.

The severe impact on email deliverability

Understanding the SPF 10-lookup limit

The SPF specification, detailed in RFC 7208, mandates a maximum of 10 DNS lookups when evaluating an SPF record. This limit is in place to prevent denial-of-service attacks that could be launched by crafting malicious SPF records designed to overload DNS servers. It’s a crucial safeguard for the stability of the internet's email infrastructure.

Mechanisms in your SPF record that trigger DNS lookups include a, mx, ptr, exists, and include. Each time one of these is evaluated, it counts towards the limit. If a domain listed in an include mechanism itself contains DNS-lookup mechanisms, those also count towards your overall total. You can learn more about how SPF a records affect DNS lookups.

When the number of lookups exceeds 10, the SPF check results in a PermError (permanent error). This means the recipient's mail server cannot properly evaluate your SPF record, often leading to email rejection or misclassification as spam. It's crucial to understand how important the 10 DNS lookups limit is.

SPF PermError: a critical problem

A PermError signals that your SPF record is invalid or unparseable due to issues like too many DNS lookups. This is a severe problem because it prevents proper email authentication, making your domain appear suspicious to receiving mail servers, regardless of your sender reputation.

Impact on Deliverability: Emails will likely be rejected or routed to the spam folder.
Domain Reputation: Consistent PermErrors can harm your domain's sending reputation, making future emails even harder to deliver.
DMARC Failure: SPF alignment, a key component of DMARC, will fail, leading to DMARC authentication failures. This undermines your efforts to protect your domain from impersonation.

Beyond delivery: brand reputation and trust

Domino effect on email authentication and security

Exceeding the SPF DNS lookup limit doesn't just affect SPF itself. It has a significant ripple effect across your entire email authentication posture, particularly on DMARC. DMARC relies on either SPF or DKIM (or both) to pass alignment checks. If SPF fails due to a PermError, your DMARC record will also fail authentication for that message, even if DKIM passes. This can leave your domain vulnerable.

The immediate consequence is that DMARC policies set to quarantine or reject might cause legitimate emails to be sent to spam or blocked entirely. This undermines the very purpose of DMARC, which is to protect your brand from phishing and spoofing attacks. For a deeper understanding of email authentication, refer to our guides.

Exceeding the lookup limit

Authentication: SPF fails with a PermError, causing DMARC to fail SPF alignment.
Deliverability: High risk of emails being rejected or going to spam, regardless of content.
Reputation: Damage to your sender reputation due to consistent authentication failures.
Security: Increased vulnerability to email spoofing and phishing attacks against your domain.

Healthy SPF implementation

Authentication: SPF passes, contributing to DMARC alignment and successful authentication.
Deliverability: Improves inbox placement by verifying legitimate sending sources.
Reputation: Strengthens your sender reputation, building trust with email providers.
Security: Provides robust protection against malicious actors attempting to impersonate your domain.

A PermError can also lead to your domain being placed on an internal blacklist (or blocklist) by receiving mail servers, making it incredibly difficult to deliver future emails. The Twilio SendGrid documentation highlights that exceeding the lookup limit returns a PermError, which can cause SPF to fail.

Resolving SPF lookup limit errors

An illustration of a hand inspecting a complex SPF record.

Practical steps for identifying and resolving the issue

The first step to resolving an SPF DNS lookup limit issue is to identify if you have one. You can use an SPF checker tool to analyze your domain's SPF record and count the number of DNS lookups. Look for a PermError related to too many lookups. DMARC reports, easily viewable through a DMARC monitoring solution like Suped, will also clearly indicate SPF authentication failures due to PermErrors. Mailhardener explains how SPF lookup limits work and their impact.

Example of an SPF record exceeding lookup limitDNS

v=spf1 include:_spf.example.com include:_spf2.example.org include:thirdparty.net include:another.com include:fifth.biz include:sixth.io include:seventh.co include:eighth.info include:ninth.app include:tenth.cloud include:eleventh.xyz -all

Resolving the issue often involves optimizing your SPF record. Common strategies include using SPF flattening, where multiple include mechanisms are resolved into their IP addresses to reduce lookups. Another approach is consolidating sending services where possible or using subdomains for different senders. You can find detailed guides on how to fix SPF record lookup limits and options for dealing with overstuffed SPF records.

Regular monitoring of your DMARC reports is essential to ensure that your SPF changes are effective and that you are not encountering further authentication issues. Tools like Suped provide real-time alerts and actionable recommendations to help you maintain a healthy email sending posture.

Ensuring optimal email delivery

Why Suped is your ally against SPF lookup limits

Maintaining a perfectly configured SPF record can be complex, especially for organizations with multiple email sending services. Suped simplifies this challenge by offering robust DMARC monitoring and SPF flattening capabilities. We provide a unified platform that brings together all your email authentication needs, giving you a clear overview of your domain's health.

Our platform's AI-Powered Recommendations don't just show you data; they tell you exactly how to fix issues and strengthen your policy, including solutions for SPF lookup limits. With Real-Time Alerts, you'll know immediately if your SPF record is causing problems, allowing for quick remediation before it impacts your deliverability. Our unified platform also integrates DMARC, SPF, and DKIM monitoring with blocklist and deliverability insights, giving you a complete picture.

For Managed Service Providers (MSPs) and agencies, our Multi-Tenancy Dashboard allows you to manage multiple domains from a single, clean interface, making it easier to monitor and optimize SPF for all your clients. With a generous free plan and a focus on simplicity, Suped makes advanced email security accessible to everyone.