You can likely do this with RM, however this was similar code that worked for me on ST. You can set start/stop times and adjust the frequency of notification (in case you have it opened multiple times a day). I also have pics of my setup and suggested devices.
Can you please describe how you connected the internal and external contacts together?
I ended up using an Iris Zigbee contact sensor for mine. Taped the contact under the box at edge and used a small round magnet and have it on the mailbox “door”. Traveling today but can post pictures this weekend if anyone is interested.
Inside there is a small connector with 2 screw terminals. You essentially connect your reed switch to those two terminals, when the connection between them is complete, the contact sensor is closed - when you disconnect the connection, it's open. You can test it with a small wire bridging those two terminals.