how to make a mail box system in minecraft with a hopper and command block?

You can do this without command blocks, just put a chest under the hopper and run a comparator from the hopper to whatever you want to notify you, like a note block or redstone lamp.

Here is a very basic circuit to notify the presence of an item. I used a chest but you can use a hopper if you like.

Comparator directly out of a chest. Place one item in the chest and comparator outputs a signal of one:

Here is the same circuit but it shows that a comparator can be used through a block. This could be used to have a mailbox outside the house and the indicator on the inside of the house.

This is the same circuit as above but there is an item in the chest and a redstone lamp: