re: MailBox

My preferred method is packed 3/4 minus gravel.
Put an inch or so in the bottom of the hole
Treat the bottom of the post. I use roofing tar
Set the post in the hole 
Pour 3 or 4 inches of gravel in on all sides
Pack it down tight with a 2x2 and a heavy hammer. Pack until it's solid
Repeat until hole is filled.

We get a lot of rain here. Posts rot easily. I have pulled out posts that I have set this way after 20 years and they look new.

The gravel allows water to drain away.