Comment by theshrike79

It's a process. I just set up a filter on Fastmail that showed me all mail that was delivered to my @gmail.com address within the last two weeks.

Then I went through that list every now and then, unsubscribing or changing email addresses as they came up. At the moment it's mostly just mail from sites I've used Google OAuth for some reason and can't be bothered to change (or just plain can't - one site actually uses email as a primary key and are completely 100% unable to change it.)