I don't think it's actually a felony to throw away junk mail that isn't addressed to you after it arrives at your house. The statute is written to address cases where mail is taken from the custody of the US Postal Service; it refers to "authorized depository of mail matter" where other language in the US Code specifically calls out letterboxes and the like. I do not believe your home mailslot qualifies as "an authorized depository". I did some searching and found some support for that belief, but nothing very authoritative (a MetaFilter thread and an eHow article).
Let me just acknowledge right away that this is trivia; the proliferation of strict-liability crimes (and the fact that strict-liability is actually a kind of default in federal law) is a real problem, as is the problem of open-ended legislation where the details are filled in by unelected regulators.