If DMARC passes but SPF fails, what are the concerns and impacts on email deliverability?

Key findings

• DMARC requirement: DMARC only requires either SPF or DKIM to pass authentication and align with the From domain. If DKIM is passing and aligned, a DMARC pass is achieved even if SPF fails.
• Impact on deliverability: If DMARC passes due to DKIM, the immediate negative impact on deliverability from SPF failure is minimal. The email should generally reach the inbox without being blocked or sent to spam based solely on this configuration.
• Common cause of SPF failure: Mail forwarding often causes SPF to fail because the forwarding server's IP address isn't included in the original sender's SPF record. DKIM, however, typically survives forwarding.
• Alignment types: SPF failure in a DMARC context often refers to a lack of alignment (the Return-Path domain not matching the From domain), rather than the SPF check itself failing (meaning the sending IP is unauthorized).

Key considerations

• Optimal configuration: While DMARC may pass, aiming for both SPF and DKIM to pass and align is generally the preferred setup for robust email authentication. This provides redundancy and strengthens your sender reputation.
• Troubleshooting SPF issues:Investigate why SPF is failing. It might be due to incorrect SPF records (e.g., missing includes, too many lookups) or legitimate mail forwarding scenarios that aren't properly handled. Consider how email forwarding affects SPF authentication.
• Perception by receivers: Some receiving mail filters might have unique configurations that could still scrutinize messages with SPF failures, even if DMARC passes. While rare for legitimate mail, a cleaner authentication record is always beneficial.
• Future-proofing: As email authentication standards evolve, having both SPF and DKIM properly configured and aligned provides a more resilient setup against potential future policy changes or stricter enforcement by mailbox providers. Ensuring proper alignment helps with overall email deliverability and avoidance of spam folders.

Key opinions

• DMARC's flexibility: Many marketers recognize that DMARC is designed to pass if either SPF or DKIM is successfully authenticated and aligned. This dual mechanism provides a safety net for deliverability.
• Focus on DKIM: If SPF is failing (especially due to forwarding), marketers often rely on DKIM to carry the authentication weight, ensuring DMARC passes and emails are delivered as expected.
• Limited negative impact: The direct negative impact on inboxing from an SPF failure (when DKIM and DMARC pass) is generally considered minimal by marketers, as the DMARC pass validates the email's legitimacy.

Key considerations

• Investigating SPF alignment: Marketers should still understand why SPF isn't aligning, even if DMARC passes. This often points to configurations by their Email Service Provider (ESP) or forwarding practices.
• Redundancy for deliverability: While one passing authentication method is enough for DMARC, having both SPF and DKIM correctly configured and aligned offers redundancy. This can be beneficial if one method unexpectedly fails or if a receiver prioritizes a specific authentication type.
• ESP limitations: Some ESPs manage the SPF domain themselves, making full SPF alignment challenging for senders. In such cases, a robust DKIM setup becomes even more critical.
• Long-term reputation: Although not an immediate deliverability blocker, maintaining optimal authentication records contributes to a stronger, long-term sender reputation. Marketers should strive for the cleanest possible authentication setup.

Key opinions

• DMARC's flexible validation: Experts confirm that DMARC successfully validates if either SPF or DKIM passes alignment checks. The preference is for both, but one is sufficient.
• Forwarding scenarios: SPF often fails when emails are forwarded because the forwarding server's IP isn't in the original SPF record. DKIM, however, typically remains intact and ensures a DMARC pass in these cases.
• No adverse deliverability impact: If DMARC passes due to a valid DKIM signature, the SPF failure usually does not negatively affect deliverability. The email is still seen as authenticated and legitimate.
• Distinction in SPF failure: It's important to distinguish between an SPF failure where the sending IP is unauthorized and one where SPF simply doesn't align with the 5322.From domain within the DMARC context. The latter is less problematic if DKIM aligns.

Key considerations

• Maintaining a clean record: While not strictly necessary for DMARC to pass, striving for SPF alignment alongside DKIM creates a more robust and tidy authentication setup.
• Addressing underlying issues: If SPF is truly failing (not just alignment in a forwarding scenario), it indicates a fundamental issue with authorized sending IPs that should be fixed by publishing the correct DNS records.
• Vendor configurations: ESPs may control SPF records, making SPF alignment difficult for senders. In such cases, DKIM becomes the primary mechanism for DMARC compliance. Understanding your ESP's setup is crucial.
• Consistency matters: While DMARC is forgiving, consistent authentication signals (both SPF and DKIM passing) build a stronger sender reputation over time, which can benefit overall deliverability and inbox placement rates, as detailed by EmailTooltester.com.

Key findings

• DMARC's core logic: DMARC checks for a valid result from either SPF or DKIM, combined with alignment of the authenticated domain with the RFC5322.From header. One successful and aligned authentication is enough.
• SPF and forwarding: SPF's mechanism is susceptible to breaking during mail forwarding because the IP address of the forwarding server changes the mail path, causing it to no longer match the original SPF record. DMARC looks at the Return-Path.
• DKIM's resilience: DKIM signatures are more robust to mail path changes like forwarding because they are tied to the email content and headers, not the sending IP, allowing them to remain valid.
• Domain alignment: The core of DMARC's effectiveness lies in domain alignment for either SPF or DKIM. If the domain that passes SPF (the Return-Path domain) or DKIM (the d=domain) matches the From domain, DMARC passes.

Key considerations

• Policy enforcement: A DMARC pass means the email adheres to the domain's defined DMARC policy (p=none, p=quarantine, or p=reject). If the policy is reject, a DMARC pass is crucial to ensure delivery. You can check a DMARC failure causes.
• Reporting: DMARC reports (aggregate and forensic) will indicate which authentication method (SPF or DKIM) passed and which failed for a given email flow. This helps identify where optimization might be possible, even if DMARC is passing. For example, understanding SPF TempError in DMARC reports.
• Best practices: While DMARC is forgiving, maintaining valid SPF records for all sending IPs and ensuring SPF alignment where possible is a best practice. It provides a more complete authentication picture and offers redundancy.
• Header visibility: Mail servers and end-users can inspect email headers. A passing DMARC with a failing SPF might look less clean than a message with both SPF and DKIM passing, even if it doesn't directly impact inboxing, as discussed by Certera.