I dug out my old multimeter, and confirmed that the + buzzer solder point is the one that’s closest to the top edge (same side as volume control) of the MAIL-1200. I try to avoid battery operated Z-Wave devices if possible. So, instead of using a door/window sensor, I used a MimoLite. I connected the NO and C of the SRD-05VDC-SL-C to connect to the + and - (respectively) signal inputs of the MimoLite. Then, I reversed the open/close status of the MimoLite device. In addition to being a DC powered device, another advantage of the MimoLite is that it responds to status changes at most once per 10 seconds. So, when the MAIL-1200 buzzes 4 times when the mailbox is opened, the event associated with the MimoLite only triggers once (granted, only responding once can be done via software if needed, but no need to write special logic in software with the Mimolite’s behavior).
Thanks, Peter for your original post.