Every .dk domain whose MX record points to t2nqt4f523hh2fe2l2hqmti3ewdhxj77zm5gebrqmqqjgb7fteba.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.