I receive large numbers of messages daily from an automated system, and I've just discovered that many of them have been marked as spam. I now have ~16,000 non-spam messages in my Gmail spam folder, and I'd like them to be marked as "not spam". This shouldn't be that hard, because all of the messages I want to save have a Gmail label ("Feedback") applied to them by a filter, so simply searching `in:spam label:feedback` returns all the messages I want to save. So far so good.

The problem is that while the "Not spam" button works for selecting a small number of messages, or even all 50 messages on the page, it disappears entirely once you click the "Select all conversations that match this search" link to select all 16,000 conversations on all pages. Obviously, at this volume, doing 50 at a time isn't feasible. What can I do here?