The DMARC aspf tag is an optional DMARC tag specifying how strictly Sender Policy Framework (SPF) checks align with your ‘From’ address. It is a crucial part of the DMARC identifier alignment process, outlined in RFC 7489 section 3.1.2.

This guide will explore what DMARC aspf is within the DMARC framework, its parameters, the most common mistakes, and the best practices for an effective aspf tag implementation.

Key Takeaways

DMARC aspf is an optional DMARC tag that indicates the SPF alignment mode. The value for the DMARC aspf can be either strict(s) or relaxed(r).
The aspf (Alignment SPF) tag in DMARC specifies how strictly the domain in the SPF check should align with the “From” address in the email header.
While strict alignment provides a higher degree of security, the relaxed alignment mode provides a more flexible experience.
Common mistakes in aspf implementation include using the wrong alignment mode and misunderstanding alignment rules.
Best practices for implementing aspf include monitoring DMARC reports regularly, aiming for gradual policy enforcement, and always keeping SPF records updated.

Understanding DMARC aspf

DMARC aspf is an optional DMARC tag that indicates the SPF alignment mode. The value for the DMARC aspf can be one of the following: strict(s) or relaxed(r). It automatically defaults to the relaxed aspf=r setting. You know the alignment has been successful when the “Mail-From” address exactly matches with the “From” address domain.

What is the Role of aspf in DMARC?

Here is a step-by-step guide on how to add or modify the aSPF tag in a DMARC policy.

First of all, you need to access your DNS management. To do so, log in to your domain registrar or DNS hosting provider.
Secondly, it is necessary to locate your DMARC record by finding the current DMARC record in your DNS settings. A typical format is the following: _dmarc.yourdomain.com.
Now you can proceed to editing your DMARC policy. To do so, change the aspf tag from aspf=s (strict) to aspf=r (relaxed). The updated version will look like this: v=DMARC1; p=none; sp=none; aspf=r;
One final step is to save the new DNS settings and congratulations, you’re done!

An example of DMARC relaxed alignment: v=DMARC1; p=reject; rua=mailto: [email protected]; aspf=r; adkim=r

An example of DMARC strict alignment: v=DMARC1; p=reject; rua=mailto: [email protected]; aspf=s; adkim=s

aspf and SPF Record

The aspf tag in DMARC records and the SPF (Sender Policy Framework) can work together to improve your email authentication. An SPF record specifies mail exchange servers authorized to send emails on behalf of your domain. The aspf tag determines how strict or relaxed the SPF alignment enforcement is.

The aspf tag enables you to meet your security needs while at the same time enabling smooth email deliverability. Strict alignment provides a higher degree of security. The relaxed alignment mode provides a more flexible experience.

DMARC aspf Parameters

As we already mentioned, aspf parameters are divided into two main categories: relaxed and strict. Each have their nuances, advantages, and drawbacks, which are detailed in the section below.

Implications of Choosing Relaxed vs. Strict Alignment

In a relaxed alignment mode, DMARC alignment is considered a match under specific conditions. This occurs when the domain in the Mail From command and the domains in other headers are an organizational match.

The relevant headers for SPF alignment include the Return-Path header (bounce email address) and for DKIM alignment it’s the DKIM signature header. As a result, in this alignment mode, even subdomains will be aligned against DMARC.

This means that if the header domain matches with any alignment requirements, it will pass DMARC authentication. This authentication occurs on the email receiver’s side.

In the strict alignment for both SPF and DKIM, the email passes DMARC authentication under specific conditions. The domain in the From header and the domains in other headers must be in exact alignment.

The relevant headers are the Return-path (for SPF) and DKIM signature (for DKIM) headers. As a result, in the case of strict alignment, subdomains will not be aligned against DMARC.

The main advantage of the relaxed mode is flexibility. The strict alignment mode is preferred by some for different reasons. It offers precision and a more robust protection mechanism.

Common Mistakes and Troubleshooting

The aspf (Alignment SPF) tag in DMARC specifies how strictly the domain in the SPF check should align with the “From” address in the email header. Misconfigurations of this tag can lead to failed DMARC enforcement or unintended email rejection. Here are some common mistakes while using the aspf tag:

1. Not specifying the aspf tag:

If the aspf tag is not included in the DMARC record, the default alignment mode is relaxed. This may not be suitable for your security needs. For example, you might need an exact match between the From header and other domains. These include the domains in the Return-path (for SPF) and DKIM signature (for DKIM) headers.

Impact: Email domains that partially match will pass alignment checks. This potentially leaves room for spoofing.
Solution: Explicitly define the alignment mode based on your organization’s requirements. You can choose between relaxed or strict modes.

2. Using the Wrong Alignment Mode:

Configuring aspf=s (strict alignment) without understanding its implications.

Impact: Emails, where the SPF-authenticated domain is not an exact match with the “From” domain, will fail DMARC checks, leading to legitimate email rejections.
Solution: Ensure you are using the right alignment mode that matches your security and email deliverability goals

3. Misunderstanding Alignment Rules:

Assuming that subdomains automatically align in strict mode.

Impact: Emails sent from subdomains will fail SPF alignment checks if aspf=s is set. Subdomains don’t inherit alignment rules for SPF.
Solution: Study the alignment rules before implementing them. You can also get in touch with experts who will take care of the process professionally.

Best Practices for Implementing aspf Tag

Monitoring DMARC reports

Monitoring DMARC reports will enable you to identify any errors and ‘catch’ any unauthorized behavior before it’s too late. You can use platforms such as PowerDMARC if you need easy-to-digest reports with actionable insights.

Gradual policy enforcement

Starting with a relaxed policy will provide you with more flexibility in the initial phase. This will help you avoid false positives that could potentially impact your email deliverability.

Understanding alignment rules and gradually transitioning to strict alignment

Start ‘light’ with the relaxed policy and gradually move toward stricter policy enforcement. This will ensure a smooth and hassle-free transition while also enhancing security. A consultation with experts in the field will enable you to meet your security goals more effectively and confidently.

Keeping SPF records updated with new sending sources and third-party vendors

SPF records do not imply a ‘set and forget’ process. Instead, you need to update your SPF records per new sending sources and third-party vendors. This will ensure you distinguish between legitimate and unauthorized sources, be they old or new.

Summing Up

In conclusion, the DMARC aspf tag is a critical yet often misunderstood element of email authentication. By understanding its parameters—strict and relaxed alignment—and implementing best practices, organizations can strike a balance between robust security and smooth email deliverability. Avoid common pitfalls, monitor DMARC reports, and update your SPF records regularly to maintain optimal performance.

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Ahona Rudra

Domain & Email Security Expert at PowerDMARC

Ahona is the Marketing Manager at PowerDMARC, with 5+ years of experience in writing about cybersecurity topics, specializing in domain and email security. Ahona holds a post-graduation degree in Journalism and Communications, solidifying her career in the security sector since 2019.

DMARC aspf Tag Explanation Guide