A defensively registered domain is a domain that is not intended for use, but is registered so no other entities can own it. Defensively registered domains are considered strong candidates to implement Domain-based Message Authentication, Reporting, and Conformance (DMARC) policies on to prevent spoofing and the sending of illegitimate mail.
For example, you may own yourcompany.com, but have defensively registered the domain yourcompany.net or yourcompany.biz. In general, any domain that is registered with the intent to prevent others from using it is considered a defensively registered domain.
