How important is the 10 DNS lookups limit on SPF records?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jun 2025
Updated 15 Aug 2025
7 min read
The 10 DNS lookup limit for SPF records is a critical, often misunderstood, aspect of email authentication. While it might seem like a minor technical detail, exceeding this limit can have significant negative consequences for your email deliverability. I've encountered many situations where domains appear to pass SPF according to some checks, yet emails still end up in spam folders or are rejected outright due to this very issue.
It’s a hard technical boundary, not merely a suggestion. Ignoring it, or even being unaware of it, can lead to your legitimate emails failing SPF authentication checks, which subsequently impacts your sender reputation and inbox placement. Understanding this limit and actively managing your SPF records to stay within it is essential for anyone serious about email deliverability.
The SPF specification, outlined in RFC 7208, explicitly states that during an SPF check, the number of mechanisms and modifiers that require DNS lookups must not exceed 10. These mechanisms include a, mx, ptr, and include. Each of these can trigger one or more DNS queries, which contribute to the overall lookup count.
The distinction between DNS lookups and DNS queries is important. An SPF record might contain multiple mechanisms, but only those that necessitate a DNS query during evaluation count towards the limit. For example, an ip4 or ip6 mechanism directly specifies an IP address range and doesn't require a DNS lookup, so it doesn't count against the limit. However, an include mechanism, which refers to another SPF record in a different domain, will trigger additional lookups, often recursively.
Understanding how these lookups accumulate is crucial. A single include mechanism from a major service provider might, in itself, resolve to more than 10 lookups due to its own nested includes. This can break your SPF record even if it seems simple on the surface, making it difficult to fix an SPF record.
Many email services and marketing platforms require you to include their SPF mechanisms. This is why it's easy to accidentally exceed the limit as you add more vendors. You can find more details on SPF lookup limits explained here.
Why the limit is critical
The primary reason for the 10-lookup limit is to prevent potential denial-of-service (DoS) attacks. If mail servers had to perform an unlimited number of DNS lookups to validate an SPF record, it could be exploited by malicious actors to overburden DNS servers and slow down email delivery globally. This safeguard is in place to protect the stability and efficiency of the internet's email infrastructure.
When an SPF record exceeds the 10-lookup limit, it results in a PermError (permanent error). This tells the receiving email server that the SPF record cannot be fully evaluated. Most mail servers, especially major ones like Google and Microsoft, will then treat the SPF authentication as a fail, or at best, an unverified state. This negatively impacts your email's chances of reaching the inbox and can contribute to your emails going to spam.
The implications for deliverability are significant. Even if your SPF record has seemingly passed in some DMARC reports, this might be due to a more lenient recipient server, or perhaps the specific email path didn't trigger the excessive lookups. However, relying on this leniency is risky, as other servers (or even the same server at a different time) may enforce the limit strictly. This highlights how broken SPF records affect email deliverability.
Common causes of SPF lookup breaches
A common culprit for exceeding the lookup limit is the inclusion of too many email sending services. Each include mechanism adds to the lookup count. When you use multiple third-party services like marketing automation platforms, transactional email senders, and CRM systems, their SPF includes can quickly add up. Sometimes, a single vendor's include can resolve to multiple nested DNS queries, unknowingly pushing you over the limit.
Another frequent issue stems from web hosting providers. Many hosting services provide a single include that itself contains a large number of nested lookups, sometimes even exceeding 10 on its own. These often include unnecessary mechanisms, like ptr (which is deprecated and should be avoided) or mx (if your MX records are not used for sending), contributing to the problem. I've seen these default configurations break SPF records right out of the box, leaving no room for other necessary includes.
Here's an example of a problematic SPF record structure:
If any of these included domains themselves have multiple nested includes, you can quickly hit the 10-lookup ceiling. This is why dealing with overstuffed SPF records is a common challenge.
Strategies for SPF optimization
To avoid exceeding the 10-lookup limit, you need a proactive approach to your SPF record management.
The problem
Complexity: Too many includes make the SPF record hard to read and manage.
PermError Risk: Exceeding the 10-lookup limit results in SPF validation failure.
Deliverability Impact: Emails might be rejected or sent to spam folders.
Solutions
Consolidate includes: Use only essential include mechanisms and remove redundant ones.
IP ranges directly: Where possible, use ip4 or ip6 mechanisms to specify IP addresses directly.
Subdomains: Utilize subdomains for different sending services, each with its own SPF record, to spread out lookups. This helps with best practices for DNS lookups.
SPF flattening: Use tools or services that automatically replace include mechanisms with the resolved IP addresses. However, be cautious with manual flattening, as IPs can change, rendering your SPF record outdated. This is how you can optimize your SPF record.
Regularly review and test your SPF record to ensure it remains compliant and effective. Tools are available to check your SPF record's lookup count and validate its syntax. Staying on top of these configurations is an ongoing task, but it's vital for maintaining strong email deliverability.
Impact on deliverability and sender reputation
Importance of the 10-lookup limit
The SPF 10-DNS lookup limit is a non-negotiable standard. Violating it can lead to a PermError, which explicitly tells the receiving server that your SPF record is invalid or cannot be processed. While some receivers might be more lenient, many will treat this as an SPF authentication failure, impacting your inbox placement and potentially causing emails to be rejected or sent to the spam (junk) folder. This is why understanding why emails fail is crucial.
A key point to remember is that SPF applies specifically to the MAIL FROM (or Return-Path) address and the HELO/EHLO identity, not the From: header visible to the end-user. This means different sending services don't necessarily need to share the same SPF-validated domain, offering an opportunity to compartmentalize your SPF records across subdomains. This strategy can significantly reduce the pressure on your main domain's SPF record.
Furthermore, being listed on an email blacklist (or blocklist) can severely impact your deliverability, regardless of your SPF setup. If your IP or domain gets blocklisted, emails from that source are likely to be rejected. While SPF issues can contribute to a blocklist listing, they are separate problems. Regularly checking your blocklist status is important alongside SPF compliance. You can learn more about blocklists in this in-depth guide to email blocklists.
Final thoughts on SPF limits
In conclusion, the 10 DNS lookup limit for SPF records is highly important. It's a fundamental rule designed to maintain the stability of the email ecosystem. Exceeding this limit leads to SPF PermErrors, which can cause your legitimate emails to fail authentication, be rejected, or land in the spam folder. This directly impacts your email deliverability and overall sender reputation.
Proactive management of your SPF record is key. This includes regularly auditing your record, removing unnecessary includes, leveraging subdomains for different sending services, and considering SPF flattening solutions that update automatically. By adhering to the 10-lookup limit, you ensure your emails are properly authenticated, which is a cornerstone of successful email deliverability and good email authentication practices. It's an ongoing effort, but one that pays significant dividends in reaching your audience.
Views from the trenches
Best practices
Always include only the bare minimum of required SPF mechanisms to avoid unnecessary lookups.
Use separate subdomains for different email sending services to manage individual SPF records.
Utilize 'ip4' or 'ip6' mechanisms directly when IP addresses are stable and known, bypassing DNS lookups.
Regularly review your SPF record for accuracy and compliance, especially after adding new services.
Common pitfalls
Relying on a single SPF record for all sending services, quickly exceeding the 10-lookup limit.
Using manual SPF flattening without a system to automatically update IP addresses, leading to outdated records.
Including 'ptr' mechanisms, which are deprecated and add unnecessary DNS lookups.
Using 'mx' mechanisms when your MX records are not the primary source of outbound email.
Expert tips
When dealing with large hosting providers, check their SPF includes carefully, as they might nest many lookups.
Consider using a service that offers dynamic SPF flattening to automate IP updates and stay within the limit.
Prioritize removing redundant 'include' statements that whitelist services you no longer use.
Remember that the 10-lookup limit applies to mechanisms that trigger DNS queries, not just the count of mechanisms.
Expert view
Expert from Email Geeks says exceeding the 10 DNS lookup limit specified in the RFC will cause mail to be SPF unauthenticated, though recipient behavior varies.
2020-09-09 - Email Geeks
Marketer view
Marketer from Email Geeks says that how strictly the RFC requirements are enforced can vary between recipients.
Google and 
Microsoft, will then treat the SPF authentication as a fail, or at best, an unverified state. This negatively impacts your email's chances of reaching the inbox and can contribute to your emails going to spam.
