What the DKIM checker shows
The checker queries selector._domainkey.domain and returns the public TXT record currently visible in DNS.
A DNS lookup is not a signature test
Finding the key confirms publication. It does not prove that a particular email was signed correctly; use the email header analyzer to inspect a received message.
Common questions
Where do I find the DKIM selector?
Open the email's DKIM-Signature header and read the value after s=. Your sending provider may also show it in its DNS setup screen.
Why does DKIM say not found?
The selector may be wrong, the record may be on a different subdomain, or the DNS change may still be cached.