Subdomain
A domain inside another, such as mail.example.com within example.com; it can carry its own DNS records, including separate SPF, DKIM and DMARC.
What it means
In DNS terms any name ending in another is its subdomain, compared by whole labels: news.example.com is a subdomain of example.com, but notexample.com is not.
Senders use subdomains to separate mail streams: marketing from news.example.com, receipts from billing.example.com. Each can have its own MX, SPF record and DKIM keys.
Under DMARC, subdomains share their organizational domain with the parent, so relaxed alignment works across them. A subdomain without its own DMARC record inherits the parent's sp= policy, or p= if sp= is absent. SPF is not inherited at all: each name that sends needs its own record.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes