"Strictly speaking, a cease and desist letter is not legally binding because the recipient can choose to ignore it. However, it is a valid legal document that establishes a precedent of a plaintiff's grievance. Usually, the letter sets a time limit for the recipient to comply with the request to desist from the offending action — or deliver an appropriate response if they believe the letter was sent in error."

Take the letter. Delete it. Engaging is the fastest way to lose the domain. Their Lawyers want you to engage and start making written statements. Don't.

"Seems like an excellent way to lose your domain in a default judgement"

Lol. Don't ignore legal paperwork or court dates, which this isn't.