Private whois services
Every domain registrar maintains a publicly viewable database that displays contact information for every registered domain. Private 'whois' services display alternative information, such as their own company data, rather than the owner of the domain. 
Detectable by Common Defenses (Yes/No/Partial): Yes
Explanation: Algorithmically possible to detect COTS service usage or use of non-specific mailing addresses (PO Boxes, drop sites, etc.)
Difficulty for the Adversary
Easy for the Adversary (Yes/No): Yes
Explanation: Commercially available or easy to set up and/or register using a disposable email account.
- Mandiant. (n.d.). APT1: Exposing One of China’s Cyber Espionage Units. Retrieved March 5, 2017.