Example of this phenomena on HN: https://news.ycombinator.com/item?id=30387833
The claims there don't cover as much as people think they do! https://news.ycombinator.com/item?id=30388857
While I've never been involved on the litigation side of patents, if I received an email like this, I'd ask for not only the patent numbers but also a detailed "mapping" of how my product is infringing. The mapping is what I had to do as a patent examiner. Just show how my product infringes on the claims. That's what would have to be done in court, after all.
If they claim
A widget consisting of 3 bars.
and my product has 4 bars [0] then I'm not infringing![0] "Consisting of" in patents means exactly. If they said "comprising" then 4 bars would infringe as they could point to any 3 of the 4. You need to know a little legalese, sure, but it's not hard.