I remember there was a thread some years back with an article complaining that you get emails immediately on sign up, but that it can take up to 10 days to stop receiving emails when you unsubscribe.

One spammer said they could use the same servers for both but when you unsubscribe you have immediately signaled that you are now losing him money. So he uses the slowest cheapest part of the stack for removal. He will never fix it and doesn't care if you get some more spam after you unsubscribe since he has done the bare minimum.

If I get a single email after I've unsubscribed I go back in my inbox and mark every single email I ever received as spam.