Do you spend your days desperately awaiting credit card bills, coupons to Pizza Hut, and reminders from your dentist's office that it's time for another cleaning? We've pared down our dependence on USPS, but for those who still get physical communications of note, Make has developed an Arduino-based mailbox mod that sends push notifications when the post is in. Back in 2005, we saw a clunky device called POSTIN that did much the same thing. Thankfully, this system doesn't require an extra gadget, instead it sends messages straight to your iPhone. The postal alert system uses a snap-action switch, connected to an Arduino sensor, to signal when your mailbox is opened. A piece of code waits for the signal and then requests a URL from a PHP-enabled server, pushing an alert to your cellphone using the Prowl iPhone app. Die-hard USPS fans can check out the instructional video after the jump.