Does SPF alignment with aspf=strict provide email deliverability benefits over aspf=relaxed?
Michael Ko
Co-founder & CEO, Suped
Published 14 Jul 2025
Updated 13 Oct 2025
7 min read
When setting up DMARC, a common question arises regarding SPF alignment: whether to use a strict policy (aspf=s) or a relaxed one (aspf=r). This choice dictates how closely the domain in the SPF-authenticated Return-Path (or Envelope From) header must match the Header From domain that recipients see. Understanding the implications of each mode is crucial for maintaining good email deliverability and protecting your domain from abuse.
While strict alignment (an aspf=s) seems intuitively more secure, it doesn't always translate to direct deliverability benefits over a relaxed policy. The primary goal of DMARC is to establish a strong authentication framework. We'll explore whether stricter SPF alignment provides tangible advantages for reaching the inbox or if relaxed alignment is generally sufficient for most senders.
DMARC leverages both SPF and DKIM for email authentication, and alignment is a core concept that ties these protocols together. SPF alignment checks if the domain used for SPF validation (the Return-Path domain) matches the Header From domain that the recipient sees in their email client. This alignment is critical for DMARC to pass. You can learn more about how DMARC works in our simple guide to DMARC, SPF, and DKIM.
Relaxed SPF alignment
Relaxed alignment, indicated by aspf=r in your DMARC record, means that the SPF-authenticated domain and the Header From domain can be the same organizational domain. This allows for subdomains. For example, if your Header From domain is example.com, an SPF-authenticated domain like mail.example.com would still pass alignment. This flexibility is often preferred, especially when using email service providers (ESPs) that send from their own subdomains or when you manage various sending systems. Most DMARC implementations default to relaxed alignment because it balances security with operational flexibility.
Strict SPF alignment
Strict alignment, set with aspf=s, demands an exact match between the SPF-authenticated domain and the Header From domain. Using the previous example, mail.example.com would fail strict alignment if the Header From domain was example.com. This mode offers the highest level of stringency. However, it requires careful management of your sending infrastructure to ensure all legitimate emails comply. You can find more information about DMARC alignment, including detailed DMARC specifications in RFC 7489.
Deliverability impact of SPF alignment
From a deliverability standpoint, strict SPF alignment (aspf=s) does not inherently provide a significant deliverability benefit over relaxed alignment (aspf=r). Mailbox providers primarily look for DMARC alignment, which can be achieved with either mode. The key is that SPF (or DKIM) passes authentication and then aligns with the Header From domain. Many high-volume senders with excellent deliverability operate successfully using aspf=r.
In fact, forcing aspf=s can introduce unnecessary complexity and potential deliverability issues. If your current setup involves ESPs or third-party senders that use subdomains for the Return-Path, switching to strict alignment could cause legitimate emails to fail DMARC authentication, leading to rejection or quarantine. This is especially true if you are unaware of all sending sources or if your email infrastructure is complex. Understanding what it means when SPF is not aligned is key.
Relaxed alignment (aspf=r) is often the recommended default. It offers a good balance between security and operational flexibility. As the Canadian Centre for Cyber Security notes, relaxed alignment is the default for DMARC implementations and generally sufficient for email domain protection.
Flexibility: Allows subdomains to pass SPF alignment, which is common when using third-party email services.
Reduced False Positives: Less likely to cause legitimate emails to fail authentication and be marked as spam or blocked (blacklisted).
Security vs. flexibility
While strict alignment provides a tighter control, its direct impact on preventing email spoofing and phishing beyond what relaxed alignment offers is often marginal from a receiver's perspective. The primary defense against spoofing comes from DMARC enforcement itself. If DMARC is set to p=quarantine or p=reject, unauthenticated emails will be handled according to your policy, regardless of whether you use strict or relaxed SPF alignment. You can find guidance on email domain protection from government cybersecurity agencies.
The choice between strict and relaxed alignment often comes down to internal policy and specific sending environments. For organizations with tightly controlled email infrastructure where all legitimate sending sources can use the exact Header From domain in their Return-Path, strict alignment might be feasible. However, this is rare for most businesses, especially those leveraging cloud-based ESPs where the Return-Path often uses a subdomain owned by the ESP, like bounces.sendgrid.net or m.mailgun.org. In such cases, enforcing aspf=s would inevitably lead to DMARC authentication failures.
A common misconception is that stricter alignment automatically translates to a better sender reputation or preferential inbox placement. However, mailbox providers are sophisticated. They assess sender reputation based on a multitude of factors, including spam complaints, engagement rates, bounce rates, and consistent authentication, not just the stringency of SPF alignment. A good sender reputation is built on consistent, legitimate sending practices, not merely strict DMARC settings.
While strict alignment can be valuable in specific niche scenarios, for most organizations, the potential benefits in terms of deliverability or increased security against external spoofing do not outweigh the operational complexities and risks of legitimate email failures.
Complex Ecosystems: Many email ecosystems, especially those with multiple ESPs, rely on relaxed alignment to function without issues.
Sender Responsibility: The onus is on the sender to ensure all legitimate sending sources can pass strict alignment, which may require significant configuration.
When to consider strict alignment
Consider implementing aspf=s only if you have a deep understanding of your entire email sending infrastructure and can guarantee that all legitimate email streams will strictly align. This might be more applicable to very large enterprises with dedicated email teams and minimal reliance on third-party senders that don't allow custom Return-Path domains.
For the majority of senders, the focus should be on simply achieving DMARC alignment, whether relaxed or strict, and moving to an enforcement policy (quarantine or reject). This provides the substantial security and deliverability benefits without the added headache. If you are starting out or experiencing deliverability issues, consider exploring why your SPF alignment is inconsistent.
It's also crucial to regularly monitor your DMARC reports. These reports provide invaluable insight into your email streams, showing you which emails are passing or failing SPF, DKIM, and DMARC checks, and from which IP addresses. This data is essential for identifying unauthorized sending and ensuring your legitimate emails are correctly authenticated. Tools for DMARC monitoring can help you manage this effectively. Many DMARC providers will provide granular reports that help you fix issues like troubleshooting DMARC reports from Google and Yahoo.
Views from the trenches
Best practices
Always ensure DMARC is implemented and actively monitored for all your sending domains, regardless of SPF alignment mode.
Use DKIM for all email streams. DKIM often provides more consistent alignment than SPF, especially with third-party ESPs.
Transition to a DMARC enforcement policy (p=quarantine or p=reject) as soon as DMARC reports show low rates of legitimate failures.
Common pitfalls
Assuming that `aspf=strict` automatically yields better deliverability or higher security than `aspf=relaxed`.
Implementing `aspf=strict` without thoroughly auditing all email sending sources, leading to legitimate emails failing DMARC.
Overlooking DKIM alignment. DKIM is often more reliable for DMARC alignment, especially with third-party senders.
Expert tips
For most senders, prioritizing consistent DMARC alignment via either SPF (relaxed) or DKIM is more important than achieving strict SPF alignment.
Focus on maintaining a good sender reputation through low complaint rates, high engagement, and proper list hygiene.
If using third-party services, configure custom sending domains (subdomains) with proper CNAME records to enable DKIM signing and SPF alignment under your own domain.
Expert view
Expert from Email Geeks says DMARC aligned with `aspf=relaxed` is generally the standard to aim for, and there's no data suggesting a deliverability benefit for `aspf=strict`.
2019-11-21 - Email Geeks
Expert view
Expert from Email Geeks says that if deliverability is already good, making changes to email sending configurations might introduce unforeseen negative impacts.
2019-11-21 - Email Geeks
Key takeaways
Ultimately, for most email senders, the answer to whether `aspf=strict` provides deliverability benefits over `aspf=relaxed` is no. While strict alignment offers a higher level of control over the precise domain match, it rarely translates into better inbox placement or enhanced protection against external spoofing compared to a properly configured DMARC policy with relaxed alignment.
The critical factor for email deliverability and security is DMARC adoption and enforcement. As long as your emails pass DMARC alignment, whether via relaxed SPF or DKIM, you are meeting the expectations of mailbox providers like Google and Yahoo. Focus your efforts on achieving consistent DMARC compliance across all your sending streams and monitoring your email blocklist status.
Does SPF alignment with aspf=strict provide email deliverability benefits over aspf=relaxed? - Technical - Email deliverability - Knowledge base - Suped
Complex Ecosystems: Many email ecosystems, especially those with multiple ESPs, rely on relaxed alignment to function without issues.
Sender Responsibility: The onus is on the sender to ensure all legitimate sending sources can pass strict alignment, which may require significant configuration.
