Sender Policy Framework

DNS/Subdomain

There's not much different between subdomains and domains. Every subdomain is also a domain, and all but one domains are subdomains. The only domain that is not a subdomain, is the root domain.

It's better to say "... is a subdomain of ...".

www.example.com. is a subdomain of example.com.
www is not a subdomain.

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Sender Policy Framework

DNS/Subdomain

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