Say I move into an apartment. And over the next months and years, I keep receiving mail for the previous tenant, whom I do not know.
This is understandably annoying. The easiest and most convenient approach for me is to just throw away his mail – but would I be breaking any law if I did so?
Do not throw away mail that is not addressed to you.
If you receive misaddressed mail, write "Not at this address" on the envelope and put it in a mailbox, or give it to the mailperson (source: United States Postal Service - Reporting / returning misdelivered mail). Also, if you contact USPS they may redirect the misaddressed mail for you.
18 U.S. Code § 1702 - Obstruction of correspondence
Whoever takes any letter, postal card, or package out of any post office or any authorized depository for mail matter, or from any letter or mail carrier, or which has been in any post office or authorized depository, or in the custody of any letter or mail carrier, before it has been delivered to the person to whom it was directed, with design to obstruct the correspondence, or to pry into the business or secrets of another, or opens, secretes, embezzles, or destroys the same, shall be fined under this title or imprisoned not more than five years, or both.
This is a concise summary of the rule in Australia.
The recipient is obliged to cross out the address mark it "return to sender no longer at this address" and post it or otherwise deliver it to the addressee.
Only Australia Post may legally destroy mail.
In the United States with USPS, this appears to depend on letter type.
"STANDARD" is what the junk mail gets mailed as (and it does not get forwarded with a Change-of-Address), whereas "FIRST CLASS" is what important and personal mail gets mailed as.
Indeed, interfering with FIRST CLASS would be a serious crime. What most postmasters appear to advise is simply put it back to the outgoing pile (even without marking it, if you have nothing to mark it with), and they'll sort it out (either delivering it again to you for one more time, to make sure it wasn't just your neighbour the letter has gotten to the prior time, or, returning to sender if the name doesn't match). Ideally, it appears that they're supposed to remember who does and does not live at a given mailbox, often making certain marks on their side of the box with the names of the individuals.
As for STANDARD junk mail, it was revealed to me by the postmaster that they never do anything with returned standard mail other than throwing it directly into the recycling bin (they're only supposed to do something if it's further marked for certain address service or some such). I've had an issue of AmEx suddenly starting to send me credit card applications to someone's obviously assumed name with my address; I was advised by a postal worker that I might as well use the prepaid envelope to send it back asking for the fictional person with my address to be removed from their list (it seems to have worked after a few times).
TL;DR: make sure to return FIRST CLASS to the outgoing pile, they'll take care of it; if it's non-first-class STANDARD junk mail and you're lazy, direct recycling is a possibility.
