I think I saw you were in Denmark ?
CIDRAM you can get a TX2 and RX2 transmitter / receiver pair ( I use them for my scoreboards with 250m range )
Use a HT12E in the mailbox, with the TE pin being shorted to ground by the microswitch, and HT12D to decode , you can latch a LED to show " youve got post " and you can connect a buzzer to the VT ( valid transmission ) pin to sound an alarm.
Sorry Arduino, youre overkiil for this project 