Every .dk domain whose MX record points to hlqzbnrcdwlciptaowcpgcjo3l5bpu2ddva3ogzpgeoqu5ok5c2a.mx-verification.google.com. Mail-server reverse lookups show which organisations share a mail provider and how widely a hosted mail platform is used. Data comes from our own scanners, which re-resolve every .dk domain in the zone roughly once a week.