You might be one of those people who got subscribed to a mailing list & getting ton’s of unwanted email, which you might have marked spam; just because you don’t need that email. However google have come upwith a smarten service which helps you to unsubscribe from these types of emails.

From Gmail’s help pages:

We don’t think you should be burdened with managing messages you don’t want to receive. We do our best to put messages in Spam when we’re pretty sure you won’t want or need them. But everyone has different preferences about the mail they want to see. You may not want to read any messages sent by a certain company or mailing list, while another Gmail user finds these same messages to be valuable.

To help solve this problem, we’re providing you with an unsubscribe tool for some messages. You’ll see the unsubscribe tool when you mark a message from particular types of mailing lists as spam. If the particular message is a misuse of a mailing list you like to receive, you can Report spam as usual. But if you never want to receive another message or newsletter from that list again, click Unsubscribe instead. We’ll send a request to the sender that your email address be removed from the list. It’s that simple!

Unscribe from GMail (GMail Help Page)

Note: this pretty old information, but lot of people are not aware of this