DNS SPF Multiple Includes: Best Practices For Email Authentication

Understanding SPF (Sender Policy Framework) Fundamentals

The Sender Policy Framework (SPF) is a critical component in the suite of email authentication protocols designed to combat email spoofing and improve email deliverability. SPF enables domain owners to specify which IP addresses or mail servers are authorized to send emails on their behalf, thus helping to prevent unauthorized entities from forging emails.

At its core, an SPF record is a specially formatted DNS TXT record published in the DNS zone file of the sending domain. This DNS TXT record contains the SPF syntax, which outlines the authorized mail servers using mechanisms such as `ip4`, `ip6`, `a`, `mx`, `include`, and `all`. Proper configuration of SPF records is vital for email server configuration and overall email security.

Microsoft, Google, Amazon Web Services, and other major email service providers advocate for the use of SPF alongside complementary technologies such as DKIM (DomainKeys Identified Mail) and DMARC (Domain-based Message Authentication Reporting and Conformance). Together, these protocols form a robust approach to email authentication, reducing the risk of email spoofing and enhancing trust in email communications.

The Role of DNS in Email Authentication

DNS plays a foundational role in email authentication by being the mechanism through which SPF and other relevant records are retrieved and evaluated. When a receiving mail transfer agent (MTA) gets an incoming email, the server consults DNS authoritative servers to locate the domain’s SPF record contained as a DNS TXT record.

The DNS resolver on the recipient’s end issues a DNS lookup query to fetch the SPF record. Due to DNS caching and DNS propagation delays, updates to SPF records may not be instantly visible globally, requiring administrators to account for DNS record TTL (Time to Live) settings. Effective DNS management and careful control of DNS zone files ensure SPF records are responsive and consistent, which improves email deliverability.

Organizations often use DNSSEC (DNS Security Extensions) in tandem with SPF to guard against DNS spoofing or cache poisoning attacks. The synergy of SPF with reverse DNS lookups, IP address whitelisting, and mail exchanger (MX) records forms a comprehensive security posture.

How SPF Records Work: A Technical Overview

An SPF record specifies policies indicating which servers can send mail on behalf of the domain. Its evaluation happens in real-time during email receipt and follows a precise procedure:

• SPF Record Lookup: The recipient’s DNS resolver sends a DNS lookup query targeting the domain’s SPF DNS TXT record.
• SPF Record Evaluation: The SPF record evaluation parses SPF syntax to match the sending IP address against defined mechanisms—`ip4`, `ip6`, `a`, `mx`, `include`, and others.
• Mechanisms and Qualifiers: The default qualifiers are `+` (pass), `-` (hard fail), `~` (soft fail), and `?` (neutral), which instruct the mail server how to treat the results.

A critical constraint is the DNS lookup limits inherent in SPF processing. SPF requires that the total number of DNS recursive lookups (including those invoked by `include` mechanisms) must not exceed 10. This restriction affects SPF record optimization since exceeding the DNS lookup limit results in an automatic SPF neutral or fail, harming email deliverability.

The TXT record limitations, including maximum length and DNS response size constraints, further complicate SPF configuration, necessitating tactics like SPF record flattening, which replaces `include` mechanisms with static IP addresses to avoid excessive DNS lookups.

What Are ‘Include’ Mechanisms in SPF Records?

The `include` mechanism in SPF allows embedding the SPF policies of a third-party domain within your own SPF record. This is indispensable for organizations that outsource some or all of their email sending to cloud providers like Twilio SendGrid, Amazon Web Services, or Mailchimp.

For example, a simplified SPF record might look like this:

v=spf1 include:spf.protection.outlook.com include:_spf.google.com -all

This record delegates authorization to Microsoft’s (Office 365) and Google’s email servers, authorizing their IPs to send on behalf of the domain.

The inclusion is recursive, meaning the policy within the included domain’s SPF record is also evaluated, and its DNS lookups count toward the DNS lookup limits. Each `include` mechanism triggers additional DNS recursive lookups, and as such, excessive SPF multiple includes can lead to the exceeded DNS lookup limit error.

SPF alignment, which is crucial for DMARC enforcement, also depends on the correct application of include mechanisms. Misconfigurations in the include mechanism in SPF can cause SPF record evaluation failures, adversely impacting email security and delivering inaccurate SPF pass/fail results.

Challenges of Using Multiple Includes in SPF Records

While SPF multiple includes enable flexible delegation of email senders, relying heavily on them raises significant challenges:

DNS Lookup Limits and SPF Record Length

Every `include` mechanism incurs DNS recursive lookups at evaluation time. Domains that use multiple third-party services such as Cloudflare, Proofpoint, or Barracuda Networks alongside in-house servers can quickly surpass the maximum allowed 10 DNS lookups. Once the DNS lookup limit is exceeded, the SPF record evaluation results in an SPF neutral or SPF fail, leading to Spam filtering or outright delivery rejection.

Additionally, SPF record length restrictions and TXT record limitations impose difficulties maintaining clean, manageable SPF records in DNS zone files. Long records may fragment across multiple DNS TXT strings, increasing the complexity of DNS management and propagation.

DNS Propagation and Caching Considerations

Frequent changes to SPF records, especially those with multiple includes, require mindful DNS record TTL settings to avoid negative effects of stale data in DNS caching. When DNS authoritative servers update SPF records, propagation delays can cause inconsistent SPF checks, impacting email security temporarily.

SPF Record Optimization Strategies

To navigate the complexities introduced by multiple includes, SPF record optimization techniques are essential:

• SPF Record Flattening: Tools offered by providers such as Valimail, OnDMARC, or Agari automate IP address whitelisting by expanding includes into direct IP addresses. This reduces the number of DNS lookup queries.
• Subdomain SPF Records: Segmenting services onto subdomains with separate SPF records can limit the scope of includes.
• Macro Expansion in SPF: Used sparingly, macros can tailor SPF evaluation dynamically but add complexity.
• Monitoring with SPF Record Troubleshooting Tools: Platforms like DMARC analyzers or Proofpoint’s email security tools can diagnose multiple SPF record issues and suggest refinements.

Multiple SPF Record Issues and Email Server Configuration

One of the common missteps is publishing more than one SPF record for a single domain, causing SPF record evaluation conflicts and unpredictable results. DNS standards specify only one SPF record per domain is valid; multiple records lead to SPF neutral response or SPF fail.

Ensuring proper mail exchanger (MX) records and reverse DNS configurations complement SPF records is part of email server configuration best practices promoted by companies such as Cisco Systems and Oracle to bolster email deliverability and reduce false positives in Spam filtering.

Large enterprises like Facebook, Twitter, LinkedIn, Yahoo, and Zoho Mail continuously refine SPF policies to balance DNS lookup constraints with effective email authentication. Collaborative efforts with DNS providers like GoDaddy, Namecheap, Bluehost, Dyn, Verisign, and Akamai ensure stable DNS infrastructure supporting SPF and related protocols.

Statistical Data: Industry-wide SPF Adoption and Challenges

• Over 90% of inbound emails to Google’s Gmail are subject to SPF checks.
• Approximately 15% of domains fail SPF record evaluation due to exceeded DNS lookup limits.
• SPF record flattening reduces DNS lookups by an average of 35% in multi-include environments.
• Organizations using multiple third-party email services report higher rates of SPF soft fail without proper SPF record optimization.
• DNS TTL settings of less than 3600 seconds reduce SPF record update propagation time by up to 50%.

Source: Google Postmaster Tools, Valimail SPF Analytics, OnDMARC Impact Report

DNS Lookup Limits and Their Impact on SPF with Multiple Includes

The Sender Policy Framework (SPF) is a critical component of email authentication, designed to combat email spoofing and enhance email deliverability. However, one of the most vital considerations when implementing SPF records—especially with multiple includes—is managing DNS lookup limits.

SPF verification involves parsing the SPF record from the domain’s DNS TXT record stored in the DNS zone file, and recursively performing DNS lookups for each included domain or IP address. The SPF specification limits the number of DNS lookups during SPF record evaluation to 10 to prevent excessive DNS query traffic, which can overload DNS authoritative servers and cause delays. This includes DNS recursive lookups caused by the include mechanism in SPF, a, mx, ptr, and exists mechanisms.

With multiple SPF includes—common when delegating email delivery to third-party services like Microsoft 365, Google Workspace, Twilio SendGrid, or Mailchimp—it’s easy to inadvertently exceed this lookup limit. Exceeding the DNS lookup limit results in a permerror, effectively causing SPF evaluation to fail and potentially triggering an SPF hard fail or soft fail, depending on the domain’s SPF policy. This negatively affects email security posture and email deliverability, raising flags in spam filtering systems operated by providers such as Proofpoint or Barracuda Networks.

DNS propagation and DNS caching at DNS resolvers help mitigate some lookup strain, but the recursive nature of includes combined with the DNS TTL settings must be carefully managed to avoid exceeding DNS response size limits or causing delays during mail exchanger (MX) delivery.

Best Practices for Structuring SPF Records with Multiple Includes

To optimize SPF records with multiple includes and comply with DNS lookup limits, several best practices should be followed:

• Minimize the Number of Includes: Use the include mechanism in SPF judiciously. Instead of stacking multiple includes for services like Amazon Web Services, Zoho Mail, and Shopify, consider consolidating IP addresses where possible.
• Flatten SPF Records: SPF record flattening involves replacing includes with the actual IP addresses or CIDR blocks, thus reducing the need for multiple DNS lookups. Tools and services such as Valimail and OnDMARC provide automated flattening solutions.
• Limit DNS TXT Record Length: The DNS TXT record has inherent size limits; excessively long SPF records with multiple includes can cause DNS response truncation. Optimizing SPF record length improves DNS response size and reduces SPF record evaluation time.
• Leverage IP Address Whitelisting: Where feasible, use IP address whitelisting in SPF using the ip4 or ip6 mechanisms instead of includes, especially for fixed IP ranges of trusted mail transfer agents (MTAs).
• Regularly Review SPF Syntax and Policies: Email administrators at companies like Google and Microsoft recommend ensuring SPF syntax is valid and compliant with SPF policies to avoid multiple SPF record issues.
• Use Subdomain SPF Records: Delegate specific email sending sources to subdomain SPF records to distribute DNS management and reduce complexity in the primary domain SPF record.
• Implement DNSSEC: Protect your DNS records against tampering, improving the integrity of SPF record evaluation by DNS resolvers.

Common Pitfalls When Implementing Multiple Includes in SPF

Several common mistakes can undermine SPF effectiveness and complicate email security posture:

• Exceeding DNS Lookup Limits: One of the most frequent pitfalls is exceeding the SPF DNS lookup limit, leading to failed SPF evaluations. This often happens with overlapping includes from multiple email service providers.
• Multiple SPF Records: Publishing more than one SPF record (i.e., multiple DNS TXT records with different SPF syntax) results in SPF parsing failures. Domain administrators using platforms like GoDaddy or Bluehost must ensure only a single consolidated SPF record exists.
• Misconfigured Include Mechanism: Incorrect placement of the include mechanism, or failing to account for macro expansion in SPF, can break SPF logic or cause unexpected SPF neutral or soft fail results.
• Ignoring DNS TTL Settings: Setting DNS record TTL too low can increase DNS resolver load, while overly high values might delay DNS propagation after SPF record updates.
• Improper SPF Record Flattening: Over-flattening SPF records can cause maintenance challenges, especially when IP ranges of third-party providers change, as seen with Fastly or Akamai networks.
• Neglecting SPF Alignment: SPF needs to be aligned properly with the domain in the “From” header, as required under DMARC to prevent passive authentication failures.
• Overlooking TXT Record Limitations: DNS TXT records have size limits—nearly 255 characters per chunk—and improperly segmented SPF records can cause invalid DNS responses.

Case Studies: Real-world Examples of Multi-Include SPF Usage

• Microsoft 365 and Twilio SendGrid Integration: Organizations commonly use Microsoft 365 for internal emails and Twilio SendGrid for marketing campaigns. Combining their SPF records involves multiple includes like `include:spf.protection.outlook.com` and `include:sendgrid.net`. Without SPF record optimization and flattening, such combinations can easily hit DNS lookup limits, causing SPF hard fail and email deliverability issues.
• E-commerce Platforms Using Shopify and Mailchimp: Many retailers use Shopify for transactional emails and Mailchimp for newsletters. The SPF record includes `include:shops.shopify.com` and `include:spf.mcsv.net`. Proper SPF syntax adherence and DNS management are critical to prevent exceeded DNS lookup limits that would degrade email security.
• Large Enterprises Using Amazon Web Services, Google Workspace, and Cisco Systems Security Products: Large enterprises often encompass many email service vendors. Amazon SES, Google Workspace, and Cisco’s Email Security solutions each require inclusion in the SPF record. To reduce DNS lookup loads, they employ SPF flattening and IP address whitelisting, supported by SPF record evaluation tools provided by entities like Valimail and Agari.

Alternatives to Multiple Includes: Using IP Ranges and Subnetting

With the limitations imposed by DNS lookup caps and DNS TXT record size constraints, alternative approaches to multiple includes can improve SPF record efficiency:

• IP Range Specification: Instead of using multiple SMTP providers’ includes, listing the IPs or CIDR subnets explicitly via the ip4 and ip6 mechanisms greatly reduces DNS lookup queries. However, maintaining the updated IP ranges is labor-intensive.
• Subnetting: Group IP addresses into subnet blocks for inclusion in SPF, minimizing the total entries and simplifying DNS zone file management.
• SPF Record Flattening Services: Companies like OnDMARC and Valimail offer SPF record flattening services where all include directives are resolved to IP addresses offline and consolidated into one optimized DNS TXT record.
• Subdomain Delegation: Employ subdomain SPF records—for example, marketing.example.com or transactional.example.com—to distribute SPF includes and IP addresses, reducing the burden on the root SPF record.
• Email Authentication Synergy (DKIM and DMARC): Complement SPF with DKIM and DMARC for better email authentication coverage, reducing dependency on SPF include complexity.
• DNS Management Best Practices: Efficient DNS management with providers such as Cloudflare, GoDaddy, or Namecheap ensures SPF record updates propagate swiftly, and DNS caching parameters remain optimal.

By leveraging these alternatives along with strict SPF record syntax adherence and DNS lookup query limits, organizations improve email security and decrease the risk of multiple SPF record issues that degrade SPF pass rates and overall deliverability.

The Impact of Multiple Includes on Email Deliverability

In the landscape of email authentication, managing a Sender Policy Framework (SPF) record with multiple includes can be both necessary and challenging. The include mechanism in SPF allows domain owners to delegate sending authority to third-party services, a common practice among organizations using email platforms like Microsoft 365, Google Workspace, Amazon Web Services (AWS), or marketing tools such as Mailchimp and Twilio SendGrid. However, SPF multiple includes bring the risk of exceeding DNS lookup limits, a critical constraint imposed by the SPF specification which caps the number of DNS lookup queries to 10 per SPF check.

This DNS lookup limit is crucial because every include mechanism triggers a DNS recursive lookup by the email server’s mail transfer agent (MTA) or the DNS resolver to retrieve the associated DNS TXT record. When this 10-lookup threshold is exceeded, the SPF evaluation fails with an SPF hard fail, or more commonly, an SPF neutral or soft fail, which negatively impacts email deliverability. These fail states increase the likelihood of legitimate emails being marked as spam or rejected by recipient mail exchangers (MX), exacerbating issues for businesses relying on bulk or transactional email.

Moreover, multiple includes complicate SPF record length and TXT record limitations, as DNS zone files have size constraints and DNS response sizes ideally should be minimized for efficient resolution and DNS caching. Long SPF records with multiple includes also delay DNS propagation and make DNS management more complex across DNS authoritative servers. Common issues encountered include multiple SPF record issues—such as having more than one SPF record per domain—which also cause SPF record evaluation failures.

Organizations like Valimail, OnDMARC, and Agari offer advanced SPF record optimization tools that help identify redundant includes and flatten SPF records, replacing complex include chains with IP address whitelisting or host mechanism entries. This approach reduces DNS lookup queries and response size, improving email deliverability and maintaining compliance with DNS lookup limits.

Tools and Resources for Managing Complex SPF Records

Navigating SPF record complexity requires robust tools and resources that facilitate SPF syntax validation, SPF record troubleshooting, and DNS management. Widely used DNS management platforms such as Cloudflare, GoDaddy, Namecheap, and Bluehost facilitate easy editing of SPF DNS TXT records and provide DNS record TTL controls, allowing domain owners to fine-tune the DNS caching behavior, crucial for propagating SPF changes efficiently.

Advanced SPF record evaluation tools are available from providers like Cisco Systems, Proofpoint, Mimecast, and Barracuda Networks. These tools simulate SPF record evaluation in real-time, detecting SPF multiple includes and highlighting exceeded DNS lookup limits or syntax errors. Valimail and OnDMARC further specialize in SPF record flattening, which resolves all nested includes into an optimized record with fewer DNS lookups while ensuring SPF pass results are maintained.

Organizations should leverage SPF record examples and best practice frameworks from entities like Microsoft, Google, and Oracle, which publish detailed guides on SPF syntax, macro expansion in SPF, and examples of correct SPF policies. These resources help streamline email server configuration and integration with mail exchangers (MX) and reverse DNS setups, enhancing overall email security.

Additionally, DNSSEC (Domain Name System Security Extensions) can be employed alongside SPF to secure DNS queries from tampering, thereby increasing the reliability of SPF record verification. Tools offered by Cloudmark, Postmark, and Akamai provide insights into email authentication standards and help diagnose spam filtering issues linked to SPF fail states.