The common confusion surrounding SPF (Sender Policy Framework) checks often revolves around which domain is actually validated. Many assume it checks the 'From' header address, but the standard protocol specifies that SPF primarily authenticates against the Return-Path domain, also known as the Mail From domain or envelope sender. Understanding this distinction is crucial for proper email authentication and deliverability, especially when using third-party sending services or subdomains.
Key findings
Primary check: SPF validation primarily occurs against the Return-Path domain (or Mail From), not the friendly 'From' header.
Null sender: For emails with a null sender, SPF checks the HELO or EHLO value during the SMTP conversation.
Third-party senders: If a third-party platform uses its own Return-Path domain, including them in your 'From' domain's SPF record offers no benefit.
Microsoft's historical approach: In the past, Microsoft sometimes checked SPF against the 5322.From (header From) domain, leading to some advising SPF records for both, but this is less common now.
DNS lookup limits: Misinterpreting SPF instructions can lead to exceeding the 10 DNS lookup limit, causing SPF failures. Learn more about the SPF record definition.
Key considerations
SPF record placement: Ensure your SPF record is published on the correct domain, typically the Return-Path domain of your sending server.
Subdomain strategy: Use subdomains for sending (e.g., send.example.com) to manage SPF lookups and avoid overburdening your primary domain.
Vendor guidance: Be cautious of third-party vendor instructions regarding SPF that may recommend unnecessary includes, impacting your DNS lookup limits.
Authentication protocols: Understand the role of SPF alongside DKIM and DMARC for comprehensive email authentication.
Domain alignment: While SPF checks the Return-Path, DMARC requires alignment between the Return-Path and the 'From' header domain for a passing result. This is covered in more detail in our article on SPF in email.
Email marketers often wrestle with the nuances of SPF, particularly when using external sending platforms. There's a persistent belief that SPF should validate the 'From' header domain, but experienced marketers emphasize that the Return-Path (or Mail From) domain is the critical point of inspection. This misunderstanding can lead to incorrect SPF configurations, potentially affecting email deliverability and even causing blocklist issues.
Key opinions
Return-path focus: Many marketers confirm that SPF solely checks the Return-Path domain, especially when third-party platforms handle bounce management.
Common misconception: A widespread misunderstanding exists where users are incorrectly told to add SPF includes for their main domain, even when a vendor uses its own Return-Path.
DNS lookup limits: Incorrect SPF setups often lead to exceeding the 10 DNS lookup limit, causing authentication failures and deliverability challenges.
Vendor updates: Some major email service providers (ESPs) have recently updated their guidance to correctly reflect SPF's behavior, advising against unnecessary includes.
Trial and error: Marketers often conduct their own testing to determine if specific receiving providers (like Microsoft) might still perform non-standard SPF checks against the 'From' header.
Key considerations
Validate SPF: Regularly check your SPF records for validity and adherence to standards. An SPF record checker can assist.
Strategic includes: Only include external senders in your SPF record if they are using your domain for the Return-Path. Avoid unnecessary SPF entries.
Stay informed: Keep up to date with changes in ISP authentication policies, as they can evolve over time.
Marketer view
Email marketer from Email Geeks clarified that SPF strictly checks against the Return-Path domain. They emphasized that if a third-party platform uses its own Return-Path for bounce handling, adding their details to your domain's SPF record would be ineffective and unnecessary for authentication.
03 Dec 2020 - Email Geeks
Marketer view
Email marketer from Email Geeks highlighted a common misinterpretation in third-party authentication guides. These often mislead users into believing they need to include every sending platform in their primary domain's SPF record, which can inadvertently cause them to exceed the 10 DNS lookup limit, leading to authentication failures.
03 Dec 2020 - Email Geeks
What the experts say
Industry experts provide clarity on SPF's authentication mechanisms, debunking common myths and offering practical advice. They emphasize that while the SPF protocol primarily checks the Return-Path domain (5321.from), historical practices and specific receiver implementations (like Microsoft's past behavior) sometimes led to confusion regarding the 'From' header (5322.from). Experts advise careful configuration to optimize deliverability and avoid exceeding DNS lookup limits.
Key opinions
Protocol standard: Experts confirm that the SPF protocol mandates checks against the 5321.from (Return-Path) domain or the HELO/EHLO value for null senders.
Microsoft's exception: In the past, Microsoft's non-standard SPF checks against the 5322.from (header From) domain influenced SPF recommendations, though this is less prevalent now.
Conditional 5322.From: Experts generally advise against publishing SPF records for the 5322.from domain unless specifically troubleshooting Microsoft delivery issues, as SPF mainly focuses on the Return-Path. This is also why our article on SPF records on both sender domains is important.
Domain separation: Incorrect SPF entry placement can be a common issue. The SPF record must be on the domain or subdomain specified in the Mail From path.
Subdomain benefits: Using sending subdomains (e.g., send.example.com) can offload DNS lookup burden from the organizational domain, helping both domains pass SPF checks within limits.
Key considerations
Return-Path domain authentication: Always prioritize configuring SPF records for your Return-Path domain to ensure proper authentication.
Microsoft deliverability: If experiencing specific deliverability challenges with Microsoft, investigate if SPF records for the 5322.from domain are necessary. For Microsoft SPF issues, consider troubleshooting.
DMARC alignment: Understand that DMARC builds upon SPF by requiring alignment between the Return-Path domain and the 'From' header domain. Learn more about the Sender Policy Framework explained.
Review configurations: Regularly review your SPF, DKIM, and DMARC configurations to ensure they are optimal and not causing unintended failures.
Expert from Email Geeks states that the SPF protocol explicitly directs SPF checks to the 5321.from domain. They add that in cases where mail has a null sender, the SPF check is performed against the HELO/EHLO value.
04 Dec 2020 - Email Geeks
Expert view
Expert from Email Geeks recalled previous discussions about Microsoft's historical SPF checking behavior. They mentioned that in the past, Microsoft sometimes checked against the 5322.from domain, which led to recommendations for SPF publishing in both the 5321.from and 5322.from domains.
04 Dec 2020 - Email Geeks
What the documentation says
Technical documentation consistently clarifies that SPF checks are performed against the domain specified in the RFC 5321 Mail From address (the Return-Path). This contrasts with the RFC 5322 From header, which is the 'friendly' address seen by recipients. Adhering to these specifications is vital for effective email authentication and for preventing your legitimate emails from being flagged as spam or outright blocked. Understanding this distinction is fundamental for anyone managing email infrastructure.
Key findings
Authoritative source: Documentation confirms that SPF authenticates against the domain found in the SMTP Mail From command (the envelope sender or Return-Path).
IP authorization: An SPF record is a DNS TXT record that lists the IP addresses and hostnames authorized to send email on behalf of a specific domain.
Verification process: Recipient mail servers use the SPF record to verify if an incoming email's sending server is permitted by the Return-Path domain's policy.
Spoofing prevention: Proper SPF configuration significantly reduces the risk of email spoofing and phishing attempts using your domain.
Complementary protocols: SPF works in conjunction with DKIM and DMARC to provide comprehensive email authentication. For an overview, see A simple guide to DMARC, SPF, and DKIM.
Key considerations
SPF record accuracy: Ensure your SPF record is accurate and includes all legitimate sending sources for the Return-Path domain.
Domain ownership: The SPF record must be published on the domain (or subdomain) that is used in the Return-Path address of your outgoing mail.
DMARC alignment impact: While SPF checks the Return-Path, for a DMARC pass, this domain must align with the header From domain (DMARC alignment). This is a critical distinction for overall email security.
Dynamic updates: Some services like SendGrid or Mailgun may provide methods for dynamic SPF updates to avoid DNS lookup limits, or offer sub-domain use.
Blacklist prevention: Proper SPF setup helps prevent your domain from being added to email blocklists or blacklists due to unauthorized sending.
Technical article
Documentation from DuoCircle specifies that an SPF record is a straightforward text record. Its primary function is to list all authorized hostnames and IP addresses that have permission to send emails on behalf of an organization's domain.
15 Oct 2021 - DuoCircle
Technical article
Documentation from IONOS Digital Guide states that SPF, or Sender Policy Framework, is a crucial method. It enables mail servers to verify whether a received email genuinely originated from the specified host server, thereby enhancing email security.