Was the mailbox white? Does it look like the color is on top of your car finish or below it? You can use a fingernail to try to gently remove a bit of the white to see what you are dealing with.
Was the mailbox white? Go to Oreilly's and ask for some rubbing compound compatible to your paint. (clear coat, etc) Apply according to directions.
Try wiping off the paint transfer with a rag sprayed with WD40. If it's paint it will wipe off.
Look at it closer. Unless you're a total dumbass, you should be able to tell if the paint is gouged or if it is just a scuff on the surface. If the paint is gouged, you should be able to feel it as well. EPIC FAIL on hitting the mailbox in the first place.
DR + Mrs Bears face
Hi so the white streaks are not something that has been added. they are where paint has been removed. it will need to be repaired and resprayed over.