When I install the 6 x 6 s for the mailbox foundation, should I use sonotubes?

Answer 1

Sonotubes work well in providing a slip resistant surface to prevent heaving if you live in an area where frost penetrates the ground.

Otherwise, the sonotube isn't really needed unless you want to ensure your excavation does not cave in on you before or while you are placing the concrete. Suggested if you are digging in sand soil.

Answer 2

Sonotubes are not needed for that but for driveways and foundations, Yes.

Answer 3

Nah, just use concrete to fill the hole. It will conform to the imperfections in your hole.