In simple terms, the "Mailbox mod" is attaching a commonly available rural style mailbox to the smoker w 3" AL flex, solid pipe, angles, etc., to act a a smoke source.
Could be 1' away, could be 10'.
Smoke is most commonly provided using
AMNPS 5" x 8" tray w pellets in mailbox.
So that gives you a "cold smoke" like for cheese in your smoker chamber.
If you want to "hot smoke" anything, Then you ALSO turn on a heat source in the chamber, as usual.
Main advantage of the mod is - extended times between having to re fill smoke fuel, (many say up to like 10 hours) and ability to cooler or cold smoke if desired, because the smoke amount does not depend on enough heat in the smoking chamber.
It kinda is a modern, and much more compact takoff of the oldddd remote fire w underground duct to smoking chamber idea.
Marc