[kccqzy]

I used to subscribe to the New Yorker. I do open their newsletters and I do not have images enabled. One day they simply send me an email asking me whether I'm still reading their newsletters, telling me they'll auto-unsubscribe me unless I click a link inside the email within X days.

I find that this is an elegant solution that solves the problem completely; if a user finds your emails valuable enough, they wouldn't hesitate to click a link once every few months to reconfirm their subscription.