iImage Information

Don't you wish you had a mailbox that was waterproof? One that would be large enough to hold small-to-medium sized packages? One with enough space to hold all your mail if you go out of town for a few days?
Do you have a dead microwave oven? Yes? Then your wish is about to come true!
Step 1Remove unneeded parts.
iImage Information

Take apart the broken microwave and remove the electrical cord and all of the mechanical innards, except the parts that belong to the door latch mechanism. Reassemble. This will make the box a lot lighter and easier to support on the mailbox pole. If you don't want to do this you can probably get away with just cutting off the electrical cord.
[EDITED TO ADD:] Here is an instructable that will explain how to do this without risking electrocution:
http://www.instructables.com/id/How-To-Take-Apart-A-Microwave
One caution, though. If you're away, always have your mail picked up on a daily basis. People will notice when you are away which could lead to break-ins.
(just kidding,i know its unique, and awesome too!)
5 stars, awesome 'ible!
It would appear that the most stringent regulations are reserved for people with curbside mailboxes: http://www.usps.com/receive/mailboxstandards.htm
Even in those cases I think it pretty likely they'd approve as long as it met all of the construction, measurement, installation and labeling criteria. A microwave is not very different from a regular mailbox from a structural standpoint.
Thanks!