Hi.
Today I installed Z-wave sensor on my mailbox (used Everspring SM103 with wired connection)
Sensor itself installed inside, and small magnetic switch installed outside (connected by wire through the door) - I even disassembled magnetic part to hide magnets inside of the mailbox cover. Looks nice and works perfect.
I’m using prowl to send push messages on my iPhone (found the info here: http://forum.micasaverde.com/index.php/topic,4508.msg58600.html#msg58600 )
As a result, I immediately notified when a postman inserts new correspondence to my mailbox by a pop-up message (which is free and much more convenient than sms or email) on an iPhone. Cool 
p.s. the magnetic switch I used: Standard Wired Door/Window Contact DC2 White
http://www.diy.com/nav/fix/electrical/security-alarms/doors_windows/Standard-Wired-Door-Window-Contact-DC2-White-10647529
Works perfectly with SM103.