The standard round-topped blue USPS mailboxes make for adequate cover against typical handgun ammunition. They're built out of sturdy steel that will certainly impede a bullet's flight. Not so good against .357 Mag or .357 Sig, 10mm, .41/.44 mags and the little 7.62x25mm at under 25 yards. But even these rounds are slowed up quite a bit by that steel.
Mailboxes shouldn't be considered cover against a rifle or shotgun slugs however.
Speaking of cover -- dumpsters make pretty good cover, especially if they're more than 1/2 full!