i live in a 3-unit apt. building with three mail boxes.  because the
mail carriers often do not pay attention to what mail they are putting
in what box, we get each other's mail.  on of the neighbors, when he
gets our mail, doesn't bring it inside as we do, but throws it in the
garbage.  we have asked him to stop, but he seems oblivious to any
concerns but his own and continues this practice.  we must now look
through the building garbage to make sure we are not missing mail. 
isn't he breaking federal laws by doing this?

Hi rach3-ga,

Until you start missing letters, the mail seems as certain as the sun
rising, doesn't it?  My sympathies.

Yes, your neighbor is running afoul of federal statutes.  Title 18,
part 1, chapter 38, section 1708 (full text at Cornel:
http://www4.law.cornell.edu/uscode/18/1708.html) reads, in part:

  Whoever steals, takes ... from or out of any mail, ... letter box,
mail receptacle, or any mail route ... any letter, postal card,
package, bag, or mail, ... Shall be fined under this title or
imprisoned not more than five years, or both

The statute makes no mention of whose letter nor whose mailbox is
involved.  As I interpret it, one cannot "steal" one's own mail so the
letter in question must belong to another.  Your letter clearly does
not belong to your neighbor, and it is being removed from a "letter
box, [or] mail receptacle" which seems to fit the statute.

If you tell me what state you are in, I may be able to find cases
where such offenders were successfully prosecuted.  I suspect though
that you might solve your problem by conspicuously posting, in large
type, this statute with key words highlighted or underlined.

Best of luck affecting a change in your neighbor's behavior!

-Haversian

this is exactly what i was looking for.

I worked as a Postal letter carrier for five years and I know this
happens, particularly on an 'auxillary' route.  You may wish to solve
the problem at the source, by getting a PO box. A less positive fix is
to contact the manager of the postal station your carrier works from.
Have them post an "advisory" on the wicket containing your building's
address, stating that the carrier must verify the mail address as he
places it in each box. A notification to the POSTAL INSPECTION service
(number in your phone book) will help also.

You may have a hard time getting help with this from the 'feds', but
your contact with be a Federal Postal Inspector.

Also, most state law covers this under a generic "Petty Larceny" or
"Grand Larceny" statute depending on the value of the mail taken.

Good luck and a great answer from haversian-ga!