This is something I have done each January but now I'm starting in December and hoping there will be positive results in the new year.
Basically, I go through all the emails I receive as they come in [and then work back through those which have piled up] and if they are from non-personal/friend&family sources, decide which I want to still receive in the new year. I then unsubscribe to as many as possible. After all, if I miss the newsletter I can always sign up again.
I have an app which is part of Mail [MailButler] which does much of the unsubscribing for me, but I also have used Unroll.me - a website that will do much the same thing. There are no doubt other ways of handling unsubscriptions, but this is what I do at the end of the year.
As for snail mail, that's a different thing to tackle, and a separate post. In the meantime, a little hint of spring to keep people smiling.