What to do with Returned Mail?

[Brandon Foken]

So, after my 2nd mailing, I have a stack of about 20ish returned envelopes that were un-deliverable. What do you do with those leads? I want to find a way to research the name and find out their current address. I figure that if my mail got returned, then so did all of my competitor's mail. Is there a service that allows you to track these people down? Would love some suggestions!