You could use a pair of SIM20 transceivers ( you dont have to do anything to use one as a transmitter and one as a receiver if you want to keep it simple )
They reckon 1500m in free air, I have tested a pair over 150m )( the length of my road ) and they were fine.
Hang a 170mm length of wire from the bottom of the mailbox as an antenna.
I don't know what trigger you will be using to detect a mail delivery, but it could bring the transmitter unit out of sleep, and send a code, and go back to sleep, so the battery should last a year or so?
I use Virtualwire for data transfer, and use the PTT output from the chip to power the transmitter ( it has plenty of warm up time bulit in ) so the only quiescent current is from the Arduino chip itself.
You could skip the Arduino ( scandal ) and use a HT12E chip, and just trigger the transmit enable pin , it will draw nanoamps in standby