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Summary

When sending emails from a subdomain, it is crucial to establish a distinct SPF (Sender Policy Framework) record for that subdomain. Unlike DMARC, which can inherit policies from the root domain, SPF records do not automatically extend to subdomains. This means that each subdomain used for sending email requires its own dedicated SPF record to properly authenticate sending sources and prevent spoofing.

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What email marketers say

Email marketers widely agree that a subdomain used for sending emails absolutely requires its own SPF record. They emphasize that while comprehensive authentication for all domains is generally a good practice, the sending domain, whether root or subdomain, is where SPF is most critical. Marketers also note that setting up proper authentication (SPF, DKIM, DMARC) for a new domain or subdomain is a relatively quick process that yields long-term benefits for deliverability.

Marketer view

Marketer from Email Geeks suggests that they typically set up SPF for subdomains to ensure comprehensive authentication. They believe in going 'overboard' to cover various ways email servers might check authentication, as small discrepancies can lead to deliverability issues. Ensuring all authentication methods are properly configured can account for subtle differences in how receiving servers validate incoming mail. This proactive approach helps to maximize inbox placement across diverse email environments.

09 Jan 2019 - Email Geeks

Marketer view

Marketer from Email Geeks states that getting a domain fully authenticated shouldn't take more than an hour and offers significant long-term benefits. The effort invested in proper setup pays dividends over time by improving email deliverability and sender reputation. This quick initial setup can prevent many future headaches related to emails landing in spam folders or being rejected by recipient servers. It's a small investment for a crucial return.

09 Jan 2019 - Email Geeks

What the experts say

Email deliverability experts concur that SPF records are required for subdomains used for email sending. They emphasize that the DNS (Domain Name System) does not inherently distinguish between a 'domain' and a 'subdomain' in the way humans often perceive them. Each fully qualified domain name (FQDN), whether it's the root domain or a subdomain, needs its own explicit DNS records, including SPF, unless a specific protocol (like DMARC) defines an inheritance mechanism. They caution against the misuse of wildcard records for SPF due to potential security risks.

Expert view

Expert from Email Geeks asserts that a subdomain needs its own SPF record. They clarify that the DNS does not inherently distinguish between a 'domain' and 'subdomain' in terms of record requirements. Consequently, unless a protocol explicitly states otherwise (such as DMARC, which can have an overarching policy), each specific domain name, including subdomains, requires its own matching record for authentication purposes. This ensures precise control over authorized senders for every sending identity.

09 Jan 2019 - Email Geeks

Expert view

Expert from Email Geeks provides a critical clarification: while DNS operates uniformly for both root domains and subdomains, protocols like DMARC might introduce specific inheritance rules. For instance, a DMARC record on a root domain can extend its policy to subdomains. However, SPF does not behave this way. An SPF record applied to a root domain does not automatically apply to its subdomains, and vice versa. Therefore, SPF must always be explicitly applied to the specific domain or subdomain used for sending emails, as it is foundational for email authentication.

09 Jan 2019 - Email Geeks

What the documentation says

Official documentation and technical guides consistently state that each unique domain or subdomain used for sending email must have its own SPF record. The underlying principle is that SPF authentication occurs at the specific domain level, as defined by the MailFrom (or Return-Path) domain. These resources clarify that SPF does not inherit policies from parent domains, unlike some other DNS-based email authentication mechanisms.

Technical article

Documentation from the SPF RFC (RFC 7208) specifies that SPF records apply to the 'MailFrom' domain (the domain in the Return-Path header), which can be a subdomain. It explicitly states that SPF processing should look for a TXT record directly at the domain in question, rather than inheriting from parent domains. This direct lookup mechanism reinforces the requirement for a separate SPF record for each subdomain used for sending. It's a fundamental aspect of how SPF is designed to verify sending sources at a granular level, ensuring that each distinct sending identity is properly authenticated.

April 2014 - RFC 7208 (SPF)

Technical article

Microsoft's documentation on email authentication frequently emphasizes that each domain or subdomain sending email to Outlook.com or Microsoft 365 must have a valid SPF record. They outline that the absence of such a record can lead to increased filtering or rejection of emails. This guidance underscores that Microsoft's systems perform explicit SPF checks for the specific domain used in the MailFrom address, including subdomains. Organizations are encouraged to ensure their DNS records accurately reflect all authorized sending IPs for every sending domain to maintain optimal deliverability to Microsoft recipients.

01 Jan 2024 - Microsoft Docs

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