Does an SPF record on the main domain protect subdomains?

No, an SPF record on your main domain (e.g., example.com) does not automatically protect or apply to its subdomains (e.g., mail.example.com). SPF records are specific to the exact domain they are published for. Each subdomain that sends email requires its own SPF record.

Why do subdomains need separate SPF records?

Subdomains need separate SPF records because SPF policy discovery works by directly querying the DNS record of the specific sending domain. If a subdomain is sending email and does not have its own SPF record, receiving mail servers will treat it as having no SPF policy, which can lead to authentication failures and increased spam scores. This is a key difference compared to how DMARC works for subdomains .

What happens if a subdomain sends email without an SPF record?

If a subdomain sends email without an SPF record, its emails will likely fail SPF authentication. This can negatively impact your sender reputation, cause emails to be marked as spam, or even be rejected by recipient mail servers. It also leaves your subdomain vulnerable to spoofing by malicious actors.

How does DMARC help with subdomain protection if SPF doesn't?

DMARC provides comprehensive protection by allowing a single DMARC record on your main domain to apply to all subdomains by default, using the sp tag. This means even if a subdomain lacks its own explicit SPF (or DKIM) record, the main domain's DMARC policy can still dictate how unauthenticated emails from that subdomain are handled. Monitoring DMARC reports is key to understanding this coverage.

Can I use the same SPF record for multiple subdomains?

You can, but each subdomain must have its own DNS TXT record pointing to that SPF policy. For example, subdomain1.example.com and subdomain2.example.com would each need a separate TXT record, even if the content of those records is identical. This adheres to the rule that SPF needs to be at the root domain of the sending identity.

Does SPF apply to subdomains by default? - SPF - Email authentication - Knowledge base

Illustration showing a main domain and several subdomains, some with SPF records and some without, representing how SPF policies are not inherited by default.

It's a common question in the world of email authentication, and one that often leads to misconceptions: does SPF (Sender Policy Framework) apply to subdomains by default? The straightforward answer is no, not directly. SPF records are designed to be explicit, meaning an SPF record published for your main domain does not automatically extend its protection to your subdomains.

This behavior differs significantly from how DMARC (Domain-based Message Authentication, Reporting, and Conformance) handles subdomains, where a policy on the organizational domain can, by default, apply to all its subdomains unless overridden. For SPF, each domain and subdomain is treated as a distinct entity in terms of its email sending authorization.

Understanding this distinction is crucial for maintaining robust email security and ensuring optimal email deliverability. Misconfiguring SPF for subdomains can lead to legitimate emails being marked as spam or even blocked, while also leaving your subdomains vulnerable to impersonation and phishing attacks.

How SPF works with subdomains

SPF operates at the specific domain level where the record is published. When an email server receives an email, it performs a DNS lookup for an SPF record associated with the sending domain specified in the email's Return-Path header. If that domain is a subdomain, the receiving server will look for an SPF record specifically published for that subdomain.

If no SPF record is found for the particular subdomain, the email server will generally proceed as if there is no SPF policy in place for that subdomain. It does not look up the parent domain's SPF record and apply it. This behavior is outlined in the SPF specification, as noted by security experts, meaning a subdomain without its own SPF record lacks the explicit sender authorization that SPF provides.

The consequence of this non-inheritance is that if you use subdomains for sending email, but only have an SPF record on your primary domain, emails sent from those subdomains will fail SPF authentication. This is why it's a critical aspect of email security and deliverability to address SPF for every sending subdomain explicitly.

Why each subdomain needs its own SPF record

Since SPF records are specific to the domain where they are published, any subdomain that sends email needs its own dedicated SPF record. This ensures that the subdomain's sending sources are properly authorized, preventing email impersonation and improving deliverability. You can learn more about this in our article on whether a subdomain needs its own SPF record.

This applies to various sending scenarios, such as when you use different subdomains for marketing or transactional emails. For instance, if your main domain is example.com and you send marketing emails from marketing.example.com, the marketing.example.com subdomain needs its own SPF record to explicitly authorize its sending sources.

Example SPF record for a subdomainDNS

marketing.example.com. IN TXT "v=spf1 include:_spf.example.com include:sendgrid.net -all"

This explicit configuration is essential because email providers, like Microsoft, emphasize that each defined domain or subdomain in DNS requires its own SPF TXT record for proper authentication and security. Failure to do so can result in authentication failures, impacting your reputation and deliverability.

Best practice for subdomain SPF

Always ensure that every subdomain you use to send email has its own SPF TXT record. This includes subdomains for marketing, transactional emails, system notifications, and any other purpose. A robust SPF setup prevents spoofing attempts and ensures your emails reach their intended recipients.

The role of DMARC in subdomain protection

Illustration depicting individual SPF shields for subdomains, and a larger DMARC shield covering the main domain and all its subdomains, showing DMARC's broader reach.

While SPF does not apply to subdomains by default, DMARC offers a more centralized approach to domain protection. A DMARC record published on your organizational domain can inherently cover all its subdomains, unless a specific DMARC record is published for a subdomain itself. We have a dedicated article on how DMARC policies apply to subdomains by default.

The DMARC sp tag, which stands for 'subdomain policy,' plays a crucial role here. If included in your main DMARC record, it explicitly defines the policy to be applied to all subdomains. This allows for fine-grained control over how subdomains are treated, even if they don't have their own DMARC records. You can explore how the DMARC sp tag affects subdomain policies in detail in our guide.

SPF subdomain handling

Explicit Requirement: Each subdomain must have its own SPF record.
No Inheritance: A parent domain's SPF record does not apply to subdomains automatically.
Failure Risk: Emails from subdomains without SPF records will fail SPF checks.

DMARC subdomain handling

Default Inheritance: A parent domain's DMARC policy applies to subdomains unless overridden.
SP Tag Control: The sp tag allows a specific subdomain policy.
Comprehensive Protection: DMARC provides a unified policy for both the main domain and subdomains.

Managing SPF and DMARC for complex setups

For organizations with numerous subdomains, managing individual SPF records can become complex, especially when considering the 10 DNS lookup limit. Exceeding this limit can cause SPF validation failures, negatively impacting your email deliverability. This is where a holistic approach to email authentication, integrating DMARC, becomes invaluable.

By implementing DMARC, you gain visibility into all email sending from your domain and its subdomains, whether SPF and DKIM pass or fail. This allows you to identify unauthorized sending sources and take action to protect your brand. Solutions like Suped provide powerful DMARC monitoring and reporting tools, offering AI-powered recommendations to fix issues, real-time alerts, and a unified platform for SPF, DKIM, and DMARC management. Our DMARC monitoring capabilities are designed to simplify even the most complex setups, including for MSPs.

Ultimately, while SPF doesn't automatically cover subdomains, a well-planned email authentication strategy that includes both SPF records for all sending subdomains and a comprehensive DMARC policy will provide the best protection against spoofing and ensure your emails reach the inbox. It's about combining the granular control of SPF with the overarching visibility and enforcement of DMARC.