Every .dk domain whose MX record points to pcb5t4typz6sshjr5z67xtcvaqunwo7dp7mslxrsqupb7do2zzia.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.