HI all I thought this was a very interesting thread. @joelw135. Here is what I did to solve the same issue. I used a magnet sensor extension with wires. So essentially the magenetic contacts are inside the mailbox and the main sensor is outside the mailbox (not shielded by metal, and therefore increasing the signal to the house. Here is a brief idea of how this looks on the doorbell application.
Idea was first put forth by @Mike_Maxwell and is brilliant. I just used it for the mailbox application.
I used the Ecolink Sensor and it is about 40ft away from the house, and the nearest repeater. Seems to work like a charm. I hope this helps you.