Summary
When a DMARC record includes aspf=s (strict SPF alignment), it requires an exact match between the domain in the RFC5322.From header and the domain used for SPF authentication (the RFC5321.MailFrom or Return-Path domain). If your SPF record is on a subdomain (e.g., sub.example.com) while your visible 'From' address uses the organizational domain (e.g., example.com), SPF alignment will fail. However, DMARC validation can still pass if DKIM authentication is successful and aligned.
Key findings
- Strict alignment: The aspf=s tag within your DMARC record demands an exact domain match between the From header and the SPF-authenticated domain (MailFrom).
- Subdomain mismatch: If the SPF record is on a subdomain, but the From header uses the main domain, SPF alignment will fail due to the strict requirement. This is because sub.example.com is not an exact match for example.com.
- DMARC flexibility: DMARC requires either SPF or DKIM to pass and align. Therefore, if DKIM passes and aligns, the overall DMARC check will still succeed even if SPF alignment fails. For more on this, see Mailgun's guide to email authentication.
- Relaxed alignment: Using aspf=r allows subdomains to align with their organizational (parent) domain. This provides more flexibility and is often suitable for complex sending infrastructures.
Key considerations
- Default alignment: If the aspf tag is omitted from your DMARC record, relaxed alignment is the default. This is generally the recommended setting for most senders. For more details on DMARC tags, refer to our guide to DMARC tags.
- Configuration complexity: Striving for strict SPF alignment with a subdomain sending setup can be overly complex and might not offer significant added security benefits compared to relaxed alignment, especially if DKIM is robustly implemented.
- DKIM importance: Always ensure your DKIM is properly configured and aligned. It acts as a critical fallback mechanism for DMARC validation, allowing your emails to pass even if SPF alignment fails. Read more about DMARC, SPF, and DKIM.
- DMARC policy: Your DMARC policy (p=none, p=quarantine, p=reject) determines how receiving mail servers handle emails that fail DMARC authentication. A stricter policy can lead to more delivery issues if alignment is not perfectly managed.
What email marketers say
Email marketers frequently encounter challenges with DMARC alignment, especially when their sending infrastructure involves subdomains. They often share practical experiences regarding the stringent requirements of strict SPF alignment versus the more forgiving nature of relaxed alignment.
Key opinions
- Strict alignment challenges: Many marketers find aspf=s overly restrictive for their diverse sending needs, particularly with transactional or marketing emails originating from various platforms.
- Relaxed alignment preference: The general consensus among marketers is to use aspf=r (relaxed alignment) to simplify DMARC compliance and reduce potential deliverability issues. This tag helps ensure DMARC passes by ignoring subdomains.
- DKIM as backup: Marketers often rely on robust DKIM implementation as a primary means for DMARC to pass, especially when SPF alignment cannot be strictly achieved due to third-party sending services or subdomain usage.
- Domain matching: Understanding the relationship between the visible From header domain and the SPF-authenticated domain is key to preventing DMARC failures.
Key considerations
- Sender reputation: Even if DMARC passes via DKIM, consistent SPF misalignment (especially under aspf=s) can still subtly impact sender reputation with some receiving mail servers. Learn how to improve domain reputation.
- Deliverability impact: Reliable inbox placement hinges on consistent DMARC passes. Marketers prioritize configurations that ensure messages reach the inbox without being blocked or sent to spam folders.
- Vendor configurations: When using third-party email service providers (ESPs), their unique sending infrastructure often dictates how SPF and DKIM are set up, which can affect alignment. This might necessitate using relaxed alignment.
- Monitoring reports: Regular review of DMARC aggregate reports is crucial for identifying any alignment issues or unexpected failures. This helps diagnose why DMARC verification failures are occurring.
Marketer view
Marketer from Email Geeks confirms that a DMARC record set to strict alignment will prevent a pass if the From: domain is different from the SPF record domain, for example, a subdomain.
Marketer view
Email marketer from Shopify Community emphasizes that for strict SPF alignment, the identifier header and the SPF domain must match precisely for the SPF policy to pass DMARC.
What the experts say
Deliverability experts underscore the precise mechanics of DMARC alignment, particularly how aspf=s impacts SPF's role in DMARC validation. They often advise on the optimal balance between security enforcement and practical deliverability.
Key opinions
- Exact match: Experts confirm that aspf=s necessitates an exact domain match between the RFC5322.From header and the SPF-authenticated domain. No subdomain variations are permitted under this mode.
- DMARC flexibility: A key point highlighted by experts is that DMARC only requires one of SPF or DKIM to pass and align. If DKIM is properly configured and aligned, DMARC will pass regardless of SPF alignment failures.
- Relaxed alignment benefits: For the vast majority of sending setups, experts recommend aspf=r because it accommodates common sending patterns involving subdomains or third-party senders without causing DMARC failures.
- Effort vs. security: Experts suggest that the substantial effort required to maintain aspf=s alignment across all sending variations often yields minimal additional security benefits over a well-implemented aspf=r approach, especially when DKIM is also in place.
Key considerations
- RFC compliance: A deep understanding of RFC 5322 (From header) and RFC 5321 (Return-Path for SPF authentication) is crucial for correct DMARC implementation and alignment. It's important to understand what RFC 5322 says.
- Domain reputation management: While DMARC validation might pass through DKIM, consistent SPF misalignment could still influence how some receivers perceive your domain's reputation, potentially impacting deliverability over time. Even if DMARC passes, bad SPF alignment can affect deliverability.
- Debugging failures: DMARC aggregate reports provide essential data for diagnosing any alignment issues, allowing senders to pinpoint exactly why SPF or DKIM may be failing.
- Holistic authentication: Experts stress that DMARC, SPF, and DKIM are interdependent protocols that must be correctly configured to work together for optimal email deliverability and security. You need SPF for a DMARC pass, or DKIM.
Expert view
Expert from Email Geeks unequivocally states that with aspf=s, strict alignment is enforced, requiring an exact match of domains.
Expert view
Deliverability expert from SpamResource suggests that choosing relaxed alignment is generally the more practical option for most senders to ensure DMARC compliance without excessive complexity.
What the documentation says
Official documentation, including RFCs and industry guides, provides the foundational rules for DMARC, SPF, and alignment. These resources clarify the distinctions between strict and relaxed alignment and their implications for email authentication success.
Key findings
- Strict alignment definition: Documentation confirms that aspf=s requires the domain found in the SPF-authenticated Return-Path (MailFrom) header to be an exact match to the RFC5322.From header domain.
- Relaxed alignment definition: Conversely, aspf=r (relaxed alignment) allows the SPF-authenticated domain to be a subdomain of the RFC5322.From header domain for a successful SPF alignment check.
- DMARC passing condition: The DMARC specification (RFC 7489) states that DMARC passes if at least one of SPF or DKIM passes its authentication and alignment checks against the RFC5322.From header.
- Subdomain DMARC records: Documentation clarifies that a subdomain can publish its own DMARC record, which takes precedence over the organizational domain's policy for emails originating from that subdomain. Explore DMARC records for subdomains.
Key considerations
- RFC 7489: This RFC is the authoritative source for DMARC, outlining its mechanisms, policy options, and alignment requirements. Referencing it provides the most accurate interpretation of DMARC behavior.
- Default behavior: If the aspf tag is absent from the DMARC record, the default alignment mode for SPF is relaxed (r). This is a critical detail for proper DMARC setup.
- Impact on DMARC policy: An SPF failure under strict alignment will lead to a DMARC SPF failure. If DKIM also fails, the DMARC policy (e.g., p=quarantine or p=reject) will be enforced by receiving servers. Our DMARC record examples offer more insight.
- Configuration tools: Many documentation sources provide examples and best practices for setting the aspf tag based on specific sending requirements and email flow architectures. For instance, AWS's SES DMARC guide details troubleshooting DMARC validation.
Technical article
Scaleway Documentation specifies that `aspf=s` sets the SPF alignment mode to strict, requiring SPF authentication to pass with an exact domain match.
Technical article
The 101domain Blog explains that DMARC provides two SPF alignment modes: strict (`aspf=s`), which demands an exact domain match, and relaxed (`aspf=r`), which allows a parent/subdomain relationship.
Related resources
11 resources
Related pages
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List of DMARC tags and their meanings
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How can I resolve DMARC verification failures when using a subdomain for email sending?
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