Looks like the old 'Sh!t-N-Split' routine went horribly wrong.
I know a guy who did that to an ex's mailbox once. Took the thing off the post in the middle of the night, crapped in it, and then put it back on the post.
Now I wonder if that is a Federal Offense? I've heard that tampering with a private mail box that is used to receive US Mail is a felony...probably if the intent is to intercept or destroy US Mail....but I wonder if filling it with a pile could be considered damaging to mail?
UPDATE: I found this
How serious is it to tamper with someone's personal mailbox (from their house)?
Is it a federal offense when someone tries to epoxy the mailbox shut?
* 4 months ago
Additional Details
Mailboxes are considered federal property, and federal law (Title 18, United States Code, Section 1705), makes it a crime to vandalize them (or to injure, deface or destroy any mail deposited in them). Violators can be fined up to $250,000, or imprisoned for up to three years, for each act of vandalism.