What is the difference between SPF authentication and alignment?

Key findings

• Authentication focus: SPF authentication checks the return-path (or Mail From) domain against the sending IP address, ensuring that the sending server is authorized by that domain's SPF record.
• Alignment focus: SPF alignment (specifically for DMARC) validates whether the domain found in the return-path header matches the from header domain.
• DMARC requirement: For DMARC to pass, either SPF or DKIM (or both) must not only authenticate but also align with the from header domain. You can learn more about this in our guide to DMARC, SPF, and DKIM.
• Independent checks: An email can pass SPF authentication (its return-path domain is authorized) but fail alignment if the return-path domain differs from the from header domain.

Key considerations

• DMARC enablement: For a DMARC policy to function correctly and protect your domain from spoofing, SPF or DKIM must achieve alignment. Without alignment, DMARC will always fail, even if underlying authentication passes. This can significantly impact deliverability and domain reputation. You can read more about DMARC alignment in detail.
• Shared IP use: When using an email service provider's shared IP address, the return-path domain often belongs to the ESP, not your sending domain. This means SPF authentication might pass for the ESP's domain, but SPF alignment with your from domain will fail. Solutions like dedicated sending domains are often needed for alignment.
• Deliverability impact: Even if SPF authentication passes, a lack of SPF alignment can lead to emails being marked as spam or rejected by receiving mail servers, especially those with strict DMARC enforcement. This is because non-aligned emails appear less trustworthy to recipients.

Key opinions

• Core distinction: Marketers frequently point out that SPF authentication confirms the sending server's identity based on the return-path domain, whereas alignment is about whether this return-path domain matches the user-visible from domain.
• DMARC dependency: There is a strong consensus that alignment is non-negotiable for DMARC to be effective. If alignment fails, DMARC will also fail, regardless of SPF authentication status.
• Sender reputation: Many marketers understand that poor SPF alignment can negatively impact sender reputation and inbox placement, leading to emails landing in spam folders or being rejected. For more on this, check out our article on bad SPF alignment's impact.
• ESP challenges: Marketers using email service providers (ESPs) often face challenges with SPF alignment because the ESP's domain is frequently used in the return-path. This can lead to SPF passing authentication but failing alignment.

Key considerations

• Prioritizing authentication: While alignment is beneficial, especially for DMARC, it is crucial that SPF authentication passes first. An email that fails authentication will likely face rejection regardless of its alignment status.
• Sender from domain: Marketers should always strive to have their from domain align with the authenticated domain for optimal deliverability and DMARC enforcement.
• Understanding reports: Marketers need to understand how different tools and reports (e.g., Google Postmaster Tools) display SPF authentication and alignment results, as inconsistencies can occur. Our guide on SPF alignment in DMARC reports can help clarify this.
• Impact on brand: Failing alignment can make an email look suspicious to recipients and receiving mail servers, even if technically authenticated. This can harm brand trust and engagement.

Key opinions

• Authentication first: Experts agree that SPF authentication must pass for any subsequent alignment check to hold meaning. Authentication establishes proof of origin.
• The 'proven' and 'matching' analogy: A common expert explanation differentiates SPF authentication as 'proven' (sender is authorized) and alignment as 'matching' (domains align). For DMARC, both 'proven' and 'matching' are required.
• DMARC enforcement: It is widely acknowledged that DMARC strictly relies on domain alignment. If SPF authenticates but fails alignment, the DMARC check for SPF will fail, potentially leading to quarantine or rejection of the email based on the DMARC policy. You can find more details about how SPF, DKIM, and DMARC work.
• Return-path domain: Experts stress that authentication specifically verifies the IP against the SPF record of the return-path domain (envelope sender), not the from header domain seen by the end user.

Key considerations

• Strict vs. relaxed alignment: DMARC allows for both strict and relaxed alignment modes. Relaxed mode permits subdomains to align with the organizational domain, offering flexibility but slightly less stringent control. Our article on relaxed domain alignment can provide more information.
• Impact of shared infrastructure: When using shared sending infrastructure (e.g., ESPs), the return-path domain often defaults to the provider's domain. This necessitates careful configuration of DKIM or using a custom return-path to achieve SPF alignment.
• Troubleshooting: Debugging SPF authentication and alignment issues requires checking both the SPF record for the return-path domain and ensuring its relationship with the from domain meets DMARC requirements.
• Comprehensive security: SPF authentication and alignment, along with DKIM and DMARC, form a layered security approach. Implementing all of these correctly provides the strongest defense against phishing and spoofing. More on this can be found at Medium's explanation of SPF alignment.

Key findings

• RFC 7208 (SPF): This RFC outlines the mechanism for SPF authentication, focusing on the Mail From (envelope sender) domain. It dictates that SPF authorizes specific IP addresses to send mail on behalf of a domain.
• RFC 7489 (DMARC): This RFC introduces the concept of domain alignment, which is essential for DMARC pass results. It explicitly states that either the SPF Mail From domain or the DKIM signing domain must align with the From header domain.
• Purpose of alignment: Documentation confirms that alignment's primary purpose is to ensure that the domain visible to the end-user (the from header) is the same domain that passed SPF or DKIM authentication, thereby preventing spoofing. For more, see the alignment factors in DMARC.

Key considerations

• Impact of non-alignment: RFC 7489 specifies that if an email fails DMARC alignment, the receiving server should apply the DMARC policy (none, quarantine, or reject) to the message, irrespective of whether SPF or DKIM passed their individual authentication checks. Our resource on why SPF alignment is inconsistent delves into this.
• Relaxed vs. strict alignment: DMARC documentation provides two modes for alignment: relaxed (subdomain matches organizational domain) and strict (exact domain match). The choice impacts the flexibility and security posture.
• Authentication vs. authorization: While SPF authenticates the sending server's authorization, alignment extends this by checking if that authorization applies directly to the domain presented to the user. This distinction is fundamental to DMARC's effectiveness.