How can I build a brick mailbox?
It occurred to me that this project might benefit from a single piece stone cap and eliminate lintel altogether.  You could incorporate a bit of slope and weather proof the structure.  A stone supply house could fashion it out of limestone.



More traditional lintel:
This window drawing is pretty analogous to a mailbox opening:

You are probably building a double wythe wall, not a veneer, so an upside down 'T' is used