Re: "like asking for email confirmation"
"Doesn't that just mean they would have got 500,000 "please confirm your e-mail address" messages rather than 500,000 "welcome" messages (which often include a "click here" to confirm)?"
Not necessarily - it depends on the frequency with which the list emails for any given list are sent out.
The 500,000 figure is given as within a week - so if that was 500,000 lists that send messages out weekly, it would have been 500,000 confirmation emails if all those lists did that. However, if these are daily lists, then the number of confirmations would be 1/7 of that if they all sent confirmations. That's a lot less.
The true number would probably lie somewhere between the two, because it'll be lists of varying frequency.
Sort of related but unrelated. I started getting 'newsletters' from target.com - of whom I have never been a customer. That''s a newsletter subscription that didn't require confirmation, right there.
They had an unsubscribe link, but I'm a stubborn old git and feel that if I don't subscribe, I should not be expected to unsubscribe. Instead I decided to repay my annoyance - I did a little digging, found a bunch of addresses for Target execs, and told my mail server to forward any emails from Target.com to these addresses.
I commented to that effect (including a suitable @mention) on Twitter, and got a reply from them saying that if I DM'd them my email address, they'd sort it. I didn't - instead I replied expressing my "repay my annoyance" sentiment.
I was getting those emails at least once a day when I did that. I haven't had a single one since - so none have even been forwarded to those exec addresses.
Guess: They searched for my name and removed me that way.