Every .dk domain whose MX record points to qmjn2ykrt4oulnp6e5e6di2m4j2m6pf65dfqc5hwy2o7v4rvmcmq.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.