Most advice about DMARC stops at the basics: publish a policy, monitor reports, and eventually enforce. But what happens when emails come from subdomains like mail.example.com or newsletters.example.com?

Let's examine how DMARC works with subdomains. Along the way, we’ll answer:

• How does a receiving server determine which DMARC policy to apply?
• When does a subdomain inherit the parent domain’s policy?
• Should you use the sp tag to manage subdomain behavior?

Before we begin, let's first understand the organizational domain (or parent domain), as DMARC depends heavily upon it.

Understanding the organizational domain

Every domain on the internet has a base domain from which subdomains are derived. DMARC relies on this concept to decide where to look for policy information.

Your base domain is called the organizational domain, and it’s not always just the last two segments of a hostname. For example, in the U.K., the entire yourdomain.co.uk is considered the organizational domain, not merely co.uk.

DMARC validators reference a public suffix list to determine the organizational domain. This list defines what counts as a "suffix" (or top-level domain) (like .com, .co.uk, etc.) and helps determine where a domain truly starts.

How receiving servers find a DMARC policy

When a mail server receives a message and wants to apply DMARC, it follows a process called policy discovery.

Here's a simplified diagram of the policy discovery process:

Here's what happens behind the scenes:

1. Extract the domain in the “From” header (called the RFC5322.From domain).
2. Look up a DMARC record at that domain.
   • If exactly one valid record is found, use it.
   • If multiple records are found, DMARC is skipped.
   • If none are found, continue.
3. Look up a DMARC record on the organizational domain.
   • Again, if one record is found, use it.
   • If none or multiple records are found, DMARC is skipped.
4. Check that the record includes a valid p tag.
   • If not, but there’s a valid rua (reporting address), the server defaults to p=none.
   • If there’s no valid p and no rua, DMARC isn’t applied.

How policies apply to subdomains

The DMARC policy (i.e.,p=none) on your organizational domain cascades to subdomains, but more specific policies can override it.

There are three scenarios to consider:

1. Subdomain publishes its own record (most specific policy)

Any subdomain that publishes its own DMARC policy (with a p= tag) will override whatever the organizational domain says, including the sp value.

In this example, mail.yourdomain.tld will follow its own policy (p=none), not the sp=reject from the organizational domain.

2. Subdomain follows the organizational domain’s sp tag (less specific policy)

The optional sp tag (short for subdomain policy) can be set on the organizational domain only. It allows you to treat subdomains differently from the organizational domain, without having to publish a DMARC record on each subdomain.

If the subdomain doesn’t publish its own record, it will inherit the sp value if it exists.

Here, the base domain enforces p=reject, but all subdomains default to p=quarantine since the sp tag has higher precedence than the p tag for subdomains.

The sp tag is useful if your main domain is locked down, but you’re still gradually ramping up enforcement on subdomains.

3. Subdomain inherits the organizational domain’s policy (default if no other policy is found)

If a subdomain doesn't have its own DMARC record, it will follow the organizational domain’s p value.

In this case, mail.yourdomain.tld and all other subdomains without DMARC records will implicitly enforce p=reject from the organizational domain.

Recommended approach: Start simple, then tighten up

If you're still testing the waters with DMARC or managing multiple systems, your best move is to start simple and slowly tighten up.

1. Start with a single policy

A safe default that covers your whole domain and subdomains looks like this:

2. Tighten your organizational domain first

Once you're confident all sending sources on your organizational domain are aligned and authenticated, you can tighten this up:

The sp=none tag means that subdomains will still use the safer none policy, but the organizational domain will enforce p=reject.

3. Tighten up all subdomains

Once you're confidence all sending sources on your subdomains are aligned and authenticated, you can tighten this up by either changing the sp tag to reject or by removing the sp tag entirely and relying on the p tag to cascade:

This configuration ensures that your primary domain and all of its subdomains are protected. If you need to make exceptions, publish explicit policies only on those subdomains.

Summary

Here's what to remember about DMARC and subdomains:

1. DMARC only checks two places: the domain in the From header and its organizational domain.
2. Subdomains inherit the organizational domain’s p= tag unless overridden — or unless sp= is specified at the organizational level.
3. sp= only works when defined on the organizational domain.
4. Avoid putting sp= on subdomains (it will be ignored), as it can only be applied to the organizational domain.