How do I fix the MXtoolbox SPF record DNS lookup limit exceeded error?
Michael Ko
Co-founder & CEO, Suped
Published 17 Apr 2025
Updated 19 Aug 2025
8 min read
The message from MXToolbox about exceeding the SPF DNS lookup limit can be quite alarming, especially if you're not deeply technical. It indicates a common problem where your Sender Policy Framework (SPF) record has become too complex, leading to email deliverability issues. This isn't just a warning, it's a critical error that can cause your legitimate emails to be flagged as spam or rejected outright by recipient mail servers. Understanding why this happens and how to fix it is essential for maintaining your email reputation and ensuring your messages reach the inbox.
The SPF specification, outlined in RFC 7208, imposes a hard limit of 10 DNS lookups. Each time your SPF record includes mechanisms like a, mx, ptr, or include, it requires a DNS lookup to resolve the IP addresses of the authorized sending servers. If your record triggers more than 10 of these lookups, it results in an SPF PermError (permanent error), meaning the SPF check fails.
The main culprit for exceeding this limit is often the indiscriminate addition of include mechanisms for every third-party service you use to send email, even if they're not sending mail on behalf of your root domain. For instance, if you use Google Workspace (formerly G Suite), HubSpot, Odoo, and SendGrid, each of these might suggest adding their include to your SPF record. This quickly escalates the lookup count.
To correctly address the MXToolbox SPF lookup limit error, you need to understand the difference between the Mail From address (also known as the Return-Path or Envelope From, referred to as RFC 5321.From) and the Header From address (the visible From address in an email client, RFC 5322.From). SPF authenticates the Mail From domain. Many third-party sending services, like HubSpot or SendGrid, typically use their own domain in the Mail From address for bounce handling and other purposes, even when the Header From address uses your domain.
Therefore, if a service is sending emails with their domain in the Mail From, you might not need to include their SPF record in your primary domain's SPF. Including it would only contribute to your DNS lookup count unnecessarily. You can often check the raw email headers to determine the Mail From domain being used. Look for the Return-Path or Mail-From header in an email sent through the service. If it's not your domain, then that specific include might be redundant on your root domain.
Let's look at an example. If your SPF record looks something like this, it could quickly exceed the limit:
Each include mechanism, and even the mx and a mechanisms (if present and resolving external domains), adds to the lookup count. When troubleshooting, the first step is to identify which services are truly sending email for your domain's Mail From address.
Resolving the MXToolbox SPF record DNS lookup limit exceeded error primarily involves reducing the number of DNS lookups in your SPF record. The most effective strategies focus on optimizing your existing record and, where appropriate, using subdomains.
Remove unused services: Go through your current SPF record and identify any include mechanisms for email service providers (ESPs) or SaaS tools that you no longer use, or which do not send email on behalf of your domain's Mail From address. For example, if Odoo is no longer in use, its include should be removed. This is often the quickest way to reduce lookups.
Utilize subdomains: For different email sending purposes (e.g., transactional, marketing, internal), consider segmenting your email traffic onto dedicated subdomains. Each subdomain can have its own SPF record tailored to the specific sending services used for that subdomain, reducing the complexity of your main domain's SPF record. For example, marketing.yourdomain.com could have an SPF record for HubSpot, while mail.yourdomain.com handles your corporate email via Google. This approach allows for separate SPF policies and ensures proper bounce handling.
Flatten your SPF record: SPF flattening involves converting multiple include mechanisms into ip4 or ip6 mechanisms by directly listing the IP addresses. This avoids lookups for the included domains, as the IPs are resolved only once during the SPF record's creation. However, this requires regular updates to your SPF record if the IP addresses of your sending services change.
Avoid mx and a mechanisms when unnecessary: If your primary email sending (e.g., your office email) isn't handled by the same servers as your website or inbound mail, consider removing the mx or a mechanisms from your SPF record. Each of these can trigger DNS lookups that count towards your limit, as explained in more detail in articles about how SPF 'a' records affect DNS lookups.
Once you've made changes, it's crucial to re-check your SPF record using a reliable tool like MXToolbox or a similar SPF checker. This ensures that your changes have resolved the DNS lookup limit exceeded error and haven't introduced any new syntax errors. Remember that DNS changes can take some time to propagate globally.
Maintaining an optimized SPF record is an ongoing process. As you add or remove email sending services, always review your SPF to ensure it remains within the 10-lookup limit. Over time, an SPF record can become too long or accumulate unnecessary entries, leading to future deliverability problems. Regularly auditing your email sending infrastructure and DNS records is a best practice.
Beyond SPF, ensuring proper DMARC and DKIM setup is also crucial for email authentication and overall email deliverability. DMARC relies on SPF and DKIM alignment, so a broken SPF record can directly impact your DMARC compliance. A comprehensive approach to email security involves configuring all three authentication protocols correctly.
Ensuring robust email authentication
Resolving the SPF DNS lookup limit issue can significantly improve your email deliverability and protect your domain's reputation. By carefully reviewing your SPF record, removing unnecessary includes, leveraging subdomains for distinct mail streams, and considering SPF flattening where appropriate, you can ensure your email authentication is robust. Remember to test your SPF record after any changes and continuously monitor your email sending practices to prevent future issues.
The MXToolbox SPF lookup limit error is a common headache for email senders. I've gathered some insights from the field to help you navigate this:
Best practices
Regularly audit your SPF record for outdated or unused includes to keep your DNS lookup count low and compliant.
Use subdomains for different email sending purposes; this allows for separate, optimized SPF records per service.
Always verify your SPF record using an SPF checker after making any changes to ensure correct syntax and lookup count.
Common pitfalls
Adding an SPF include for every third-party service without checking if it sends mail on behalf of your domain's Mail From address.
Failing to remove SPF mechanisms for services that are no longer in use, leading to an unnecessarily long and complex record.
Ignoring the SPF 10-DNS-lookup limit, which can result in emails being rejected or sent to spam folders by recipient servers.
Expert tips
Consider SPF flattening services for very complex setups, but be aware of the need for regular updates if IP ranges change.
For services that send from their own Mail From domain, you generally do not need to include them in your root domain's SPF record.
A well-structured SPF, combined with DKIM and DMARC, forms a strong email authentication foundation.
Marketer view
Marketer from Email Geeks says it's common to add SPF include records for every ESP or SaaS tool to the root domain, but it's often unnecessary.
March 1, 2024 - Email Geeks
Expert view
Expert from Email Geeks says digging deeper into nested includes isn't the most effective way to solve the lookup limit. The primary solution involves identifying and removing genuinely unnecessary top-level includes.
March 1, 2024 - Email Geeks
Final thoughts on SPF optimization
Tackling the MXToolbox SPF lookup limit error is a critical step in optimizing your email deliverability. It's a technical challenge, but with a clear understanding of SPF mechanisms and best practices, you can simplify your record and avoid common pitfalls. By ensuring your SPF record is lean and accurate, you're not just fixing an error, you're strengthening your email authentication and improving your chances of reaching the inbox every time.
Remember that a clean SPF record contributes to better sender reputation and helps prevent your emails from ending up on a blocklist (or blacklist). Proactive management of your DNS records is key to long-term email success.
HubSpot, while mail.yourdomain.com handles your corporate email via 
Google. This approach allows for separate SPF policies and ensures proper bounce handling.
MXToolbox or a similar SPF checker. This ensures that your changes have resolved the DNS lookup limit exceeded error and haven't introduced any new syntax errors. Remember that DNS changes can take some time to propagate globally.
