Today, let's define the term "fully qualified domain name," aka FQDN.
It can seem like a bit of a complicated term for a simple thing. What's an FQDN? It's a whole internet address. Sometimes it might be called a subdomain (I'm guilty of this), or an internet hostname. Think of it as the server name or host name + the domain name. For www.spamresource.com, www would be the hostname or server name, and then spamresource.com is the domain name. So www.spamresource.com is an FQDN, a fully qualified domain name.
If somebody asks you if you have a DMARC record for the FQDN email.spamresource.com, you would specifically enter "email.spamresource.com" in a DMARC record lookup tool. This could be called checking for a DMARC record at the FQDN level or at the subdomain level.
In the case of sending from a subdomain, like email.spamresource.com, that subdomain would also be considered an FQDN, for purposes of looking up DNS records at exactly that level of the subdomain, and also looking for specific A, MX or TXT records in DNS at exactly that level of the subdomain (email.spamresource.com), not at the top or organizational level of the domain (spamresource.com).
Don't forget to check out the DELIVTERMS section here on Spam Resource, where we define the common terms used in email technology and deliverability.
