It's a scam. Nothing more than a feeble attempt to goad you into registering a .cn domain name so they can make a few bucks.
Nothing at all to worry about. They'll try a few times before they forget about you. The last time they emailed me, I actually replied and told them to go right ahead and register whatever they want. Problem solved.
Do a Google search using the company name "WWW DOMAINBOX GROUP CO.LTD" and see for yourself:
WWW DOMAINBOX GROUP CO.LTD - Google Search