Unsubscribing from all those damn newsletters (mine included, if it bothers you) is probably the act of downsizing with the biggest bang for the buck.
You can do it manually, or there are several apps/programs available that do it for you. I have personally used Unroll.me and love it. It allowed me to unroll from 32 newsletters in about as many seconds. When you sign up for it, it takes a few minutes to scan your email account and see what you're subscribed to. Then it lists them in alphabetical order with a delete option for each and just like that, click, click, click, you're unsubbed.
They do send you a daily "Roll Up" with all of your remaining subscriptions combined in one email. I suppose that can be a good thing, but if it annoys you, just unsubscribe from it as well.
I promise this one task of unsubscribing will truly make a difference in your day to day email interactions.