What this checks

Receivers verify a DKIM signature against a public key published at selector._domainkey.yourdomain.com. This finds those keys and checks each one the way a receiver reads it: that it parses, that it is long enough, that it has not been revoked, and that nothing in the record tells receivers to ignore it.

A published, valid key is not the same as mail that passes DKIM. That also needs your provider to sign with it, using your domain. To see what a real message was signed with, check its headers.