I honestly don't know (as an avast user), there could be many ways they could obtain your email address. From someones hacked systems email address list, from a hacked site of which has your email address, your email address being hacked,etc.
I don't know how they could identify your ISP from your connection and then how your ISP could identify it was you. Yes it is technically possible, but time consuming.
There have also been scams very similar to this where the would try to extort money. So I would advise caution in any response, e.g. how do you know it is your ISP as those details could also be harvested.