What is the maximum length of an SPF TXT record string?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 Nov 2024
Updated 31 Oct 2025
7 min read
When you're dealing with email authentication, one question that often comes up is about the maximum length of an SPF TXT record. It's a critical detail because an improperly configured SPF record can seriously impact your email deliverability. Understanding the technical limits helps you prevent issues before they arise, ensuring your legitimate emails reach the inbox.
The primary rule many people recall is the 255-character limit. However, this isn't the whole story. While individual strings within a DNS TXT record are indeed capped at 255 characters, DNS allows for multiple strings to be concatenated, effectively extending the total length of your SPF record. This mechanism is crucial for domains that send email through many different services.
Getting this right is vital for maintaining your sender reputation and avoiding email being flagged as spam or rejected outright. Let's dive deeper into these limits and explore how to manage your SPF records effectively to stay compliant and ensure your emails are delivered as intended.
The DNS string limit for SPF records
The fundamental 255-character string limit
At its core, the maximum length of a single string within a DNS TXT record, including those used for SPF, is 255 characters. This limit is defined by RFC 1035, the foundational specification for the Domain Name System. When an SPF record (which is a type of TXT record) is created, each segment of text is treated as a separate string if it's enclosed in quotation marks. If you try to create a single quoted string that exceeds 255 characters, most DNS providers will reject it.
This hard limit means that if your SPF record contains many include mechanisms or IP addresses, you'll quickly bump up against this barrier. It’s a common pitfall for organizations that use multiple third-party email services, such as marketing platforms, CRM systems, and transactional email providers. Each service often requires its own include statement, contributing to the overall length.
What the RFCs say
While RFC 1035 sets the 255-character limit per string, RFC 7208, which specifically details SPF, reiterates that a single TXT record can comprise multiple strings. These strings are concatenated when the record is retrieved, forming one logical SPF record. This is how providers manage SPF records that are seemingly longer than 255 characters.
If you find your SPF record approaching this limit, it's a good time to review its structure. An overly long record can become difficult to manage and prone to errors. Sometimes, a long SPF record is also a symptom of an underlying issue, such as an excessive number of DNS lookups, which presents its own set of challenges.
Using multiple strings to extend SPF record length
Overcoming the string limit with multiple TXT strings
The good news is that while each individual string in a TXT record can't exceed 255 characters, a single TXT record can contain multiple strings. DNS resolvers are designed to concatenate these adjacent strings, treating them as one continuous record. This is the standard method for accommodating SPF records that, in their entirety, are longer than 255 characters.
For example, if your SPF record is "v=spf1 include:spf1.example.com include:spf2.example.com ~all", and it exceeds 255 characters, you can split it into two or more quoted strings. Most DNS hosting providers, such as AWS Route 53, will automatically handle the concatenation. You simply enter the SPF record as separate quoted segments within the same TXT record entry.
While this allows for very long SPF records, some DNS providers might have an overall TXT record length limit, typically around 4000 characters or limited by the UDP packet size of 512 bytes for a single DNS query. It's always a good idea to consult your DNS provider's documentation to understand their specific limitations. For most organizations, this multi-string approach effectively resolves the immediate character length issue.
Beyond character count: the 10 DNS lookup limit
The critical 10-DNS-lookup limit
Even with the ability to concatenate multiple strings, the most significant hurdle for SPF records isn't usually the character length, but the 10 DNS lookup limit. Each mechanism in your SPF record that requires a DNS lookup (like include, a, mx, and ptr) counts towards this limit. Exceeding it results in a PermError (Permanent Error), which instructs receiving mail servers to treat your email as unauthenticated, often leading to spam folders or rejection.
This lookup limit is particularly tricky because some include statements themselves can trigger multiple nested lookups. For instance, including a large ESP's SPF record might consume several lookups on its own. It's easy to unknowingly exceed this limit, especially as your organization grows and uses more email-sending services. This is why regular monitoring and optimization of your SPF record are crucial.
The problem: overstuffed SPF records
Impacts deliverability: Emails might be rejected or sent to spam if SPF fails due to too many lookups or an overly long record.
Management complexity: Difficult to update and maintain manually, leading to potential errors.
Security vulnerability: Improperly configured SPF can open doors for spoofing and phishing attacks.
The solution: SPF flattening and monitoring
SPF flattening: Dynamically resolves all include mechanisms into IP addresses, reducing lookups to one.
Continuous monitoring: Track changes and ensure your record remains compliant and effective.
Centralized platform: Use a platform like Suped for complete DMARC monitoring, SPF flattening, and deliverability insights.
To effectively manage this, tools offering SPF flattening can be invaluable. These services dynamically resolve all your include mechanisms into a single, compact SPF record composed of IP addresses. This ensures you always stay within the 10-lookup limit, regardless of how many services you add.
The importance of SPF health for deliverability
Ensuring optimal SPF implementation
Properly managing your SPF record length and DNS lookups is fundamental to email deliverability. If your SPF record is too long or exceeds the lookup limit, receiving mail servers will often fail SPF authentication. This can cause your legitimate emails to be marked as spam, quarantined, or even outright rejected. Monitoring your DMARC reports is essential to identify these failures and take corrective action.
We've developed Suped to simplify this complex aspect of email security and deliverability. Our platform offers comprehensive DMARC monitoring, alongside SPF and DKIM insights, to help you understand your email authentication status at a glance. We provide actionable recommendations, powered by AI, to help you fix issues like SPF record too long or exceeding DNS lookup limits and strengthen your overall email authentication policy.
With our real-time alerts and unified platform, you can proactively address potential problems and ensure your email program performs optimally. Suped also offers SPF flattening directly within the platform, making it easy to manage complex SPF records without manual intervention. This helps prevent deliverability issues and protects your domain from spoofing and phishing attempts.
Key takeaways
In conclusion
The maximum length of an SPF TXT record string is a nuanced topic. While a single string is limited to 255 characters, DNS allows for multiple strings to be concatenated within one TXT record, effectively circumventing that particular hurdle. However, the more critical and often overlooked limit is the 10 DNS lookups, which, if exceeded, will cause SPF to fail and impact your email deliverability.
Staying compliant with these technical specifications is paramount for ensuring your emails reach their intended recipients. Regularly reviewing and optimizing your SPF record, along with monitoring your email authentication protocols like DMARC and DKIM, will protect your sender reputation and improve inbox placement rates.
Tools like Suped are designed to make this process straightforward, offering SPF flattening, comprehensive DMARC reporting, and AI-driven recommendations to keep your email infrastructure healthy and secure. With the right approach, you can easily navigate the complexities of SPF and achieve excellent email deliverability.
