Non-existent subdomains are less protected
URGENTWhat it means
Mail claiming to come from subdomains that do not exist is handled with np=none, weaker than the sp=none you apply elsewhere. Invented subdomains are the easiest thing in the world to spoof.
How to fix it
Set np=reject. No legitimate mail comes from a subdomain that does not exist.
Check your own domain
This page describes the condition. To find out whether your domain has it, run the DMARC test or the full checkup, which examines every record at once.
Understand it properly
This page covers one condition. For the whole picture, The DMARC guide, from a first record to enforcement, or start at the Health Library.
Where this comes from
We judge this against RFC 9989 section 4.7. Every finding on this site cites the specification behind it so you can check the work rather than take our word for it.
Other DMARC conditions
DMARC_WEAKER_NP
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.