Should DMARC checks focus on SPF HELO or Return-Path and should you focus on DKIM or SPF?
Matthew Whittaker
Co-founder & CTO, Suped
Published 20 Jul 2025
Updated 17 Aug 2025
7 min read
When setting up DMARC for your domain, a common point of confusion arises: whether SPF checks should prioritize the HELO domain or the Return-Path domain. This distinction is crucial because it directly impacts how your emails are authenticated and perceived by receiving mail servers. Understanding this nuance is key to achieving optimal email deliverability and robust protection against spoofing.
Another area of debate is whether to rely more heavily on DKIM (DomainKeys Identified Mail) or SPF (Sender Policy Framework) for DMARC alignment. Both are foundational email authentication protocols, but they operate differently and offer distinct benefits when integrated with DMARC. Making the right choice, or more accurately, leveraging both effectively, can significantly strengthen your email security posture.
This discussion often becomes critical when dealing with diverse email sending infrastructures, third-party services, and varying interpretations by email security gateways, such as the one encountered with Trend Micro. Getting these configurations right ensures that legitimate emails are delivered reliably, while unauthorized messages are blocked or quarantined, helping to safeguard your brand's reputation and recipient inboxes.
SPF primarily checks if the sending IP address is authorized to send email on behalf of a domain. This check can be performed against two different domains within the email's metadata: the HELO/EHLO domain (the domain used in the SMTP greeting) or the MAIL FROM domain, also known as the Return-Path or Envelope From domain. While the SPF RFC (RFC 7208) allows for checks against both, the prevailing standard for DMARC alignment focuses on the Return-Path.
For DMARC to pass via SPF, the Return-Path domain must match the organizational domain of the Header From address (the address recipients see). This is known as SPF alignment. If an email is forwarded or sent via a third-party service that changes the Return-Path, SPF can break, even if the initial SPF check against the sending IP passes. This is a common reason why legitimate emails might fail DMARC, particularly with forwarded messages.
Most major mail providers and anti-spam systems primarily consider the Return-Path domain for SPF alignment within the DMARC context. This ensures that the domain responsible for handling bounces and other mail system messages is also verified. While some systems might check the HELO domain, relying on it for DMARC alignment is not typical practice and can lead to unexpected DMARC failures, as seen in certain specific vendor implementations.
Best practices for SPF alignment
To ensure your SPF passes DMARC checks consistently, focus on proper Return-Path alignment. This means that the Return-Path domain, often set by your Email Service Provider, should align with your Header From domain. Always review your DMARC reports to identify any SPF alignment failures and adjust your sending configurations accordingly.
While SPF verifies the sending server's IP, DKIM provides a cryptographic signature that verifies the content of the email hasn't been tampered with and that the email truly originated from the signed domain. For DMARC, DKIM alignment requires that the domain used in the DKIM signature's 'd=' tag matches the organizational domain of the Header From address. If this alignment holds, and the DKIM signature is valid, DMARC passes via DKIM, regardless of SPF status.
In the ongoing discussion of whether to focus on DKIM or SPF, many experts lean towards DKIM due to its resilience. SPF can be easily broken by forwarding, which changes the Return-Path, leading to SPF alignment failures. DKIM, however, is much more robust because its signature is embedded in the email headers and remains intact even if the email is forwarded. This makes DKIM a more reliable authentication method for DMARC, especially in complex email flows.
Ideally, both SPF and DKIM should be implemented and aligned to pass DMARC checks. Having both significantly strengthens your domain's authentication, providing redundant verification mechanisms. If one fails, the other can still ensure DMARC passes, offering better overall protection against spoofing and phishing attempts. This dual approach provides the most comprehensive email authentication.
Feature
SPF
DKIM
Primary function
Verifies sending IP address based on Return-Path domain.
Verifies sender identity and message integrity with a cryptographic signature.
DMARC alignment
Return-Path domain must align with Header From domain.
DKIM signing domain (d= tag) must align with Header From domain.
Resilience to forwarding
Breaks easily with email forwarding or mailing lists.
Generally resilient as the signature travels with the email.
Complexity of setup
Relatively simpler, single DNS TXT record.
Requires managing selectors and private keys, more complex.
Navigating DMARC challenges and vendor interpretations
Even with correct SPF and DKIM setup, DMARC can fail if a receiving mail system interprets authentication protocols differently. This highlights a significant challenge in email deliverability: the varying enforcement and interpretation of DMARC by different providers and anti-spam solutions. While most follow the DMARC RFC's emphasis on Return-Path for SPF and DKIM for overall alignment, some legacy systems or specialized security products (such as Trend Micro products) may have unique quirks, such as checking SPF alignment on the HELO domain.
When facing such issues, your best course of action is to meticulously review DMARC reports for insights into why authentication is failing. These reports provide invaluable data on SPF and DKIM pass/fail rates and alignment status. You may discover that a specific vendor is the anomaly, requiring direct communication or an adjustment to your DMARC policy (e.g., from p=reject to p=quarantine) for a transition period.
Ultimately, the goal is to ensure your DMARC policy is robust enough to protect against abuse while not blocking legitimate emails. Continuous monitoring and a flexible approach to DMARC implementation are essential. If a vendor's DMARC checking methods are causing significant issues, it might be necessary to evaluate their service or engage their support to understand their rationale and seek solutions that align with industry best practices.
Common DMARC alignment issues
SPF failure on forwarded emails: Return-Path changes, leading to SPF alignment breaking.
Third-party senders: ESPs or vendors may not properly align SPF or DKIM to your domain.
Strict HELO checks: Some receivers might focus on HELO domain for SPF, causing misalignment.
DKIM signature errors: Incorrect setup or message modification can invalidate DKIM.
Configure SPF for Return-Path: Ensure your SPF record includes all legitimate Return-Paths.
Use DMARC monitoring: Analyze reports to identify specific failing sources and reasons.
Vendor collaboration: Work with ESPs or security vendors to correct alignment issues.
Views from the trenches
Best practices
Always prioritize DKIM alignment for your DMARC policy because it is more robust against email forwarding.
Ensure SPF records are correctly configured to include all legitimate sending IPs and focus on Return-Path domain alignment.
Regularly monitor your DMARC aggregate and forensic reports to identify any unexpected authentication failures.
Communicate with third-party sending services to ensure they support DMARC alignment for your domains.
Common pitfalls
Over-relying on SPF checks against the HELO domain, which is less common for DMARC alignment.
Not having a robust DKIM setup, making your DMARC policy vulnerable to forwarding breaks.
Implementing DMARC with a 'reject' policy without first analyzing reports for potential legitimate failures.
Neglecting to monitor DMARC reports, leading to unawareness of authentication issues.
Expert tips
If a vendor enforces SPF alignment on the HELO domain, ensure your HELO domain is aligned with your From domain, or push for DKIM pass.
For optimal email authentication, always aim for both SPF and DKIM to pass DMARC alignment checks.
When troubleshooting DMARC failures, differentiate between SPF and DKIM failures to pinpoint the exact issue.
Use DMARC 'p=quarantine' initially to test and identify issues before moving to 'p=reject'.
Expert view
Expert from Email Geeks says that if the DKIM domain is aligned, it should be the primary focus for troubleshooting DMARC failures, especially if SPF issues are being misattributed.
2024-07-03 - Email Geeks
Expert view
Expert from Email Geeks says that SPF RFC allows for HELO or Return-Path checks, but DMARC RFC states HELO SPF identity is not typically used for DMARC context. Return-Path alignment is important.
2024-07-03 - Email Geeks
Key takeaways for robust email authentication
For DMARC checks, the primary focus for SPF alignment should be the Return-Path (or MAIL FROM) domain, not the HELO domain. While SPF itself permits checking either, the DMARC specification emphasizes the Return-Path for its alignment check with the Header From domain. This ensures that the domain responsible for handling bounces also passes authentication, which is a widely accepted standard across most email systems.
When considering whether to prioritize DKIM or SPF, the answer lies in leveraging both. However, DKIM often offers greater resilience, especially in scenarios involving email forwarding, as its cryptographic signature remains intact. If you must choose a primary focus for DMARC compliance, ensure your DKIM implementation is robust and aligned, as it provides a more stable foundation for authentication.
Navigating the complexities of DMARC means understanding both the standards and how various vendors implement them. Continuous monitoring through DMARC reports is crucial to identify and address any discrepancies or failures. By maintaining vigilance and a proactive approach to your email authentication, you can ensure your emails consistently reach the inbox, protecting your domain from unauthorized use and enhancing your overall deliverability.
