My main piece of advice is to get an attorney, preferably one who has experience in your industry.
They know more about how this stuff really works than you or I ever will, and you won't be able to judge what risk you're really taking without that knowledge.
But, if I were operating "without a net" like that (which I would never do!), and I really felt that the C&D was about something I wasn't doing wrong, I'd be inclined to ignore it. There's chance that you'll end up being sued, though, so you should be prepared for that possibility. That means you'll need an attorney anyway, and it will cost more than consulting one about a C&D to begin with.
> They know more about how this stuff really works
That's really the key. I used to work for a very small company that happened to have an attorney as one of the owners (there were a lot of cooks in that particular kitchen!). One day he came to me with a letter from a large RAM manufacturer demanding payment for a license on some technology I don't even remember. His only question was "do we use anything they make as far as you know?"
"Nope."
"OK. It's just a fishing expedition." Balls up the letter, throws in the trash and walks away.
They know more about how this stuff really works than you or I ever will, and you won't be able to judge what risk you're really taking without that knowledge.
But, if I were operating "without a net" like that (which I would never do!), and I really felt that the C&D was about something I wasn't doing wrong, I'd be inclined to ignore it. There's chance that you'll end up being sued, though, so you should be prepared for that possibility. That means you'll need an attorney anyway, and it will cost more than consulting one about a C&D to begin with.