Don't listen to Jespah, think about it, if everyone who received junk mail sent it back in at the cost of the sender, eventually they'd begin to see the cost effect their bottom line. It's not that it wastes energy -- it wastes money. The sender's money. Tons of junk mail being shipped twice!! Doubling the costs of the companies who send it.
And besides, like Jespah said, it mostly gimps & 'tards opening the stuff, many of them will probably be thrilled at all the pretty colors falling out of the return envelopes. You'd be making their day.
And Jespah, let's focus on reality for a moment. Junk mail existed long before telemarketing or spam; and people have hated it since about the second week. But nothing's been done. So I say, let's have a little fun at someone else's expense and avoid having to deal with the stuff. It takes all of 2 minutes to restuff the envelopes, time I now have free because I'm not on the phone w/telemarketers!!!, and I walk past my mailbox every morning anyway, so no effort to just pop something in.
And don't worry for 1 second about recycling all that crap. (1) how do you think stuff gets recycled, in factories that create pollution!!! and (2) it is a statistical fact that there are more trees in the United States now than when the pilgrims landed at Plymouth Rock. So we're OK on that front. The recycling cause is silly, it only serves as (1)political posturing to help municipal govt reps look like they're accomplishing something "meaningful" (2) it gives the nitwit hippies a way to socialize when they aren't stinking up some stadium or field flailing to Phish (3) the average citizen's elf-esteem gets a shot in the arm every time they separate their green glass from clear glass from cardboard cos they're helping Mother Earth.