Hundreds of GOV.UK domains managed by the U.K. government are on DNS-based blacklists creating email communication problems. The Domain Name System-based Blackhole List (DNSBL) is a decentralized system with different servers around the world keeping tabs on IPs spamming email users. These lists can then be surveyed via simple DNS lookup queries by mail servers to determine the probability of an incoming email message being spam. Having a domain on a DNSBL is problematic for an organization, all the more for government agencies.
Source: https://www.bleepingcomputer.com/news/security/over-400-govuk-domains-found-on-spam-blacklists/