I left this question here awhile back but got no response, so here goes again:
I've discovered that what looks like computer-generated email addresses have been used to sign up for the list. So far, I've had nearly 400.
Apparently, Jetpack only checks that the format of the email address is 'legal' but doesn't actually check if it exists. I've checked a number at random and they're all fake. But why sign up if the list is never going to get delivered, unless it's part of some other, devious objective? (I had a serious DDoS attack a week or so ago that went on for several days.)
So, firstly, how do I delete them? I've already had to remove the Email widget and move back to using my old Feedburner link. And why doesn't Jetpack check that it's a real address?