Google's Gmail service lets you powerfully search all your messages extremely fast.
Here's an easy way to quickly find emails that you may want to stop receiving.
1) go to Gmail's search box
2) type "to:me unsubscribe"
3) go through the emails it finds for a fast way to unsubscribe from the mailing lists that you're on.
This simple search can help you find services that you likely didn't request in the first place but that offer a simple unsubscribe link.
There are other services that will help automate this even further for you, but they will sometimes use your mail for research purposes, so read their terms of service and "how we offer this for free" disclaimers. Check out https://unroll.me for more information.