How can I optimize my SPF record to stay within the lookup limit when using multiple email sending services?

Key findings

• DNS lookup limit: SPF records are limited to a maximum of 10 DNS lookups. Exceeding this often results in a PermError, leading to authentication failures. Some email systems might pass emails if the IP is found before the 10th lookup, but relying on this is not a best practice.
• Vendor requirements: Many email sending services (ESPs) or SaaS products provide setup instructions that include their SPF mechanism for your domain. It is important to verify if your domain's SPF record truly needs to authorize these services by checking the Return-Path (or RFC5321.From) domain of the emails they send on your behalf. If the Return-Path domain is not yours, you may not need their SPF include.
• SPF flattening: This technique involves resolving all nested include mechanisms and replacing them with the actual IP addresses or 'a' records of the sending servers. This reduces DNS lookups to a minimum, helping to stay within the limit. For more on this, read What is SPF flattening?.
• Subdomain strategy: Assigning specific subdomains for different email sending services is an effective method to manage SPF records. This isolates the SPF lookups for each service to its respective subdomain, preventing the main domain's SPF record from exceeding the limit. This also helps with issues when troubleshooting SPF authentication issues.
• Consolidation: Avoid having multiple SPF records for the same domain. Merge them into a single TXT record to prevent validation issues. Check How important is the 10 DNS lookups limit?.

Key considerations

• Regular review: Periodically review your SPF record to remove unnecessary includes or outdated entries. Vendors may change their infrastructure, or you might stop using a service, making old entries obsolete.
• Impact of a and mx mechanisms: The a and mx mechanisms in SPF records also count towards DNS lookups. Use them judiciously. See How do SPF 'a' records affect DNS lookups?.
• SPF macros: Consider SPF macros for advanced scenarios, though they require careful implementation. They offer flexibility in dynamic SPF record creation.
• Testing: Always test your SPF record after making changes to ensure it's valid and within limits. You can use online SPF checkers to validate your record.
• Holistic authentication: While SPF is critical, ensure proper DKIM and DMARC configurations are also in place for robust email authentication. For more, refer to A simple guide to DMARC, SPF, and DKIM.

Key opinions

• Direct failure: Many marketers report that exceeding 10 lookups can lead to outright SPF failures, either for the 11th lookup and beyond or for the entire record, making it invalid.
• Vendor advice: A common sentiment is that some vendors provide incorrect or unnecessary SPF advice, leading to bloated records. Marketers advocate for verifying the Return-Path domain to see if an SPF include is truly needed.
• Old entries: SPF records often accumulate old, unused includes from past services, contributing to lookup limit issues. Regular cleanup is crucial.
• Subdomain importance: Using subdomains for different services is a preferred method for marketers to manage the SPF lookup limit effectively, as it isolates the SPF records.

Key considerations

• Validate necessity: Before adding an SPF include, confirm if the vendor's mail flow for your domain actually requires it by checking the RFC5321.From or Return-Path address. If it's not your domain, SPF may not be necessary for that vendor.
• SPF flattening: Marketers frequently consider SPF flattening services to consolidate IP addresses and reduce DNS lookups for complex setups.
• Proactive management: A proactive approach to SPF record management, including periodic audits, can prevent deliverability issues related to lookup limits.
• Understanding vendor setups: Some services, like Shopify, may use subdomains for their SPF authentication, negating the need for a direct include on your main domain. This is important to verify with all your vendors.

Key opinions

• Invalid record: An SPF record with more than 10 lookups is generally considered invalid, regardless of where the needed IP is in the lookup chain. Relying on an IP being found before the limit is not a reliable strategy.
• Unnecessary lookups: The primary cause of exceeding the lookup limit is often the inclusion of unnecessary SPF mechanisms, frequently due to outdated or misinformed vendor recommendations.
• Primary domain focus: Experts stress the importance of ensuring that your domain SPF only lists IPs that are genuinely needed, specifically those sending email with your domain in the Return-Path (RFC5321.From) address.
• Deliverability impact: Excessive lookups directly contribute to authentication failures, which negatively impact email deliverability, increasing the likelihood of messages landing in the spam folder.

Key considerations

• SPF flattening solutions: Experts recommend exploring SPF flattening services to address lookup limits, as they simplify complex SPF records into their resolved IP addresses.
• Subdomain migration: Migrating some sending services to use dedicated subdomains is a robust solution for managing SPF lookups and isolating authentication issues.
• Audit sender's return path: It is critical to check the 5321.From / return-path / bounce domain for all email senders before deciding to add their SPF include. This ensures you only authorize what is necessary.
• Consolidation: Avoid fragmented SPF records. All legitimate SPF entries for a domain should be consolidated into a single TXT record for optimal performance and compliance.

Key findings

• RFC 7208 mandate: The SPF specification (RFC 7208) explicitly limits DNS lookups to 10 to prevent denial-of-service attacks on DNS servers.
• PermError consequence: If an SPF record requires more than 10 DNS lookups during validation, the result is a PermError, indicating an invalid SPF record.
• Mechanism count: Mechanisms like a, mx, ptr, exists, and include all count towards the 10-lookup limit.
• SPF flattening definition: This is defined as the process of simplifying SPF records by replacing included domains with their IP addresses, thereby reducing the need for DNS lookups.

Key considerations

• Avoiding PTR: The ptr mechanism is deprecated and should be avoided due to its high lookup cost and potential for abuse. It also counts as a DNS lookup.
• Consolidate include statements: If multiple includes point to the same underlying IPs, they can often be combined or optimized to reduce lookups.
• Subdomain delegation: Delegating email sending to subdomains allows for separate SPF records, which effectively circumvents the main domain's lookup limit. This is a widely recommended best practice.
• TXT record length: Beyond lookups, be mindful of the 255-character limit for a single TXT record string. While related, it's a separate concern from the lookup limit. More on this at Why is my SPF record too long?.