I'm afraid that if my mail was delivered to my door I'd eventually get a message that I'd have to pick it up at the PO in the future. My 25# Orange Male Tabby doesn't back down for anything. My Son brought his Labrador over one day and the Cat had it cornered in a matter on minutes.
Funny thing is how it, and my 11# Shih-Tsu are such close "bud's". I don't think I could do a lot better for a "guard animal" if I had a doberman or German Shepherd. The cat is far more "stealthy".
BTW, I wonder if the cat's owner trained it to only reject Junk Mail and let the checks on through?