What kind of paint would I use to paint a new aluminum mailbox?
Any metal paint can be used to do the job. Most paints that you can use to paint the roof of homes is the best paint to use. I am not sure about the brand names in your area but all you need to do is go to the hardware store and look for metal based paints. These paints are designed to protect the aluminum from weather and rust.
Using Rust-olium paint and primer is the best for this. Use this spray, then use a spray sealer, to seal (Rust- olium makes sealer also).
Rustoleum is great because you don't even need to prime it.