Mart UK - ya, my main point earlier. They would have to defend their trademark in each country individually. I think generally, courts in most countries respect the patents and trademarks of others, but when you make no attempt whatever to make it publicly known that you have a patent or trademark, it would be very hard to defend in the US. Especially since there were some very long periods when nothing seemed to be happening with Norton.
I can't imagine how it works in the UK - you go to the patent office, register a trademark, stick it in your back pocket, and expect people to know you have it
Or, as I understand it (don't quote me - I don't really know) , you transfer the old company name from a Canadian company to a US holder (who had it for 15 years?) to a reborn UK company that failed and was bought by a company in India and then expect the world to know you have it?
Also, what is/are their trademark(s)? The logo on their web site is the one that includes "motorcycles" underneath. We haven't seen the letter, I wonder what exactly they are being told to quit using. Green Blob? Green Stylized Blob, Stylized Norton in several ways with and without "motorcycles", in what colors, and so on. It would be real interesting to know what is registered with various governments around the world, if anything.
BTW, I get a couple of "official" emails a year telling me to cease using gregmarsh.com and Greg Marsh Enterprises. I got the domain name in the 90s from the InterNic before it was privatized. The "official" emails offer to allow me to continue using it for a fee. They are obviously scams since the US Government assigned me the domain name and I have continuously used it since. My company is is also registered, but not a registered trademark so someone in a different state (or country) could begin using it.