When you set up Sender Policy Framework (SPF) for your email domain, a common question that comes up is whether the SPF record needs to be placed directly at the root domain. It might seem like a simple yes or no answer, but the reality is a bit more nuanced, especially when you consider subdomains and the overall email authentication landscape.
An SPF record is a type of DNS TXT record that lists all the servers authorized to send email on behalf of your domain. Its primary purpose is to help receiving mail servers verify that an incoming email from your domain originates from an authorized source, thereby preventing email spoofing and improving deliverability. Proper placement and configuration are critical for this process to work effectively.
Let's explore where SPF records should reside, how they interact with different parts of your domain structure, and what best practices to follow to ensure your emails are authenticated correctly and reach their intended recipients.
The scope of SPF records
The scope of SPF records
Yes, an SPF record primarily needs to be published at your root domain. This record will apply to emails sent from that root domain. However, a common misconception is that this single SPF record will automatically cover all your subdomains too. This is not the case.
SPF records are specifically designed to apply to the domain (or subdomain) where they are published. This means if you have multiple subdomains that send email, each of them needs its own SPF record if you intend for emails sent from them to be SPF authenticated. Without a dedicated SPF record, a subdomain defaults to having no SPF policy, which can lead to deliverability issues.
Root domain SPF
Primary record: Located at yourdomain.com and applies to emails sent directly from this domain.
Single record: You should only ever have one SPF record for your root domain. Multiple records can cause issues.
DNS lookups: The record will specify authorized IPs and included third-party sending services.
Subdomain SPF
Independent records: Each subdomain, like mail.yourdomain.com, requires its own SPF record.
Specific sources: The SPF record for a subdomain should list only the authorized sending sources for that specific subdomain.
If you manage email for a complex organization, understanding this distinction is crucial. Failing to publish SPF records for subdomains that send email can result in those emails being flagged as suspicious, leading to higher spam rates or complete rejection by receiving mail servers.
Creating and managing SPF records
Creating and managing SPF records
When creating an SPF record, you'll need to compile a list of all IP addresses and third-party services that are authorized to send email on behalf of your domain (or subdomain). This list forms the basis of your SPF record. The record typically starts with v=spf1 and includes mechanisms like a, mx, ip4, include, and a final mechanism like ~all or -all. Remember that an SPF record can only have up to 10 DNS lookups, so managing includes carefully is key.
Example SPF record for a root domainDNS
yourdomain.com. IN TXT "v=spf1 ip4:192.0.2.1 include:spf.mailsender.com ~all"
For subdomains, the process is similar. You would create a separate TXT record specifically for sub.yourdomain.com, listing only the sending sources authorized for that subdomain. If a subdomain does not send email, it's a good practice to still have an SPF record for it, often with a v=spf1 -all policy, to explicitly state that no servers are authorized to send mail on its behalf. This strengthens your overall domain security.
Regularly reviewing your SPF records is essential. As your email infrastructure evolves, you'll add or remove third-party services, requiring updates to your SPF. Neglecting these updates can lead to legitimate emails failing SPF authentication, or worse, unauthorized senders being able to spoof your domain. Consider utilizing SPF flattening services to manage the 10-lookup limit imposed by SPF, especially if you use many third-party email providers.
The role of DMARC in subdomain protection
The role of DMARC in subdomain protection
While SPF records are critical for direct domain authentication, DMARC (Domain-based Message Authentication, Reporting, and Conformance) provides a more comprehensive layer of protection. One of the key advantages of DMARC is its ability to apply policies to subdomains, even if they don't have explicit SPF or DKIM records.
A DMARC record, typically published at the root domain, can include a sp tag that specifies a policy for all subdomains. This means that even if a subdomain lacks its own SPF record, the DMARC policy from the root domain can still instruct receiving servers on how to handle unauthenticated emails from that subdomain. This is particularly useful for preventing spoofing on non-sending subdomains.
Suped and DMARC subdomain protection
Suped provides powerful DMARC monitoring tools that help you understand the authentication status of your root domain and all its subdomains. Our AI-powered recommendations help you strengthen your DMARC policy across your entire domain structure, including implementing specific policies for subdomains via the sp tag. You can also view comprehensive reports that show if subdomains need their own DMARC records or if they are covered by the root policy.
Conclusion: comprehensive email authentication
Conclusion: comprehensive email authentication
In summary, an SPF record is indeed primarily set at the root domain, but its coverage does not automatically extend to subdomains. Each subdomain that sends email requires its own dedicated SPF record for proper authentication. For subdomains that do not send email, a minimal SPF record with a hard fail policy (v=spf1 -all) is a recommended security measure.
Implementing a robust email authentication strategy involves not just SPF, but also DKIM and DMARC. DMARC, in particular, offers an overarching policy that can cover subdomains and provide valuable reporting on authentication failures, helping you maintain a healthy email ecosystem. This comprehensive approach ensures that your emails are trusted and delivered successfully, protecting your brand from spoofing and improving your overall sender reputation.
By diligently configuring and monitoring these records, you create a stronger defense against email fraud and ensure your messages consistently reach the inbox. Tools like Suped can simplify this complex task, offering insights and guidance to keep your email authentication in top shape.
