There are a lot of variables when it comes to putting a sensor in your mailbox, material of the mailbox and material of your walls would be the first ones. I know there have been some who have modified some of the older sensors, not sure if anyone has done the newer ones.
I will tag a couple people who may be able to give you a better answer on what you want to do. @Mavens@victormaletic .
Ideal conditions is by far not a mailbox, especially if it is made of metal (more of a faraday cage then). Ideal conditions is more like an open field, with both the transmitter and receiver in open air, and in clear sight of one another. Personally, I wish they wouldn’t publish ideal conditions as a spec.
But there are people who have been successful at this. And not. Here are some discussions for expectations and ideas. If the thread is before 2021, then they are probably talking about the old V1 sensor system (in fact I think all these threads use the old V1 sensors, but it should still give you an idea of the installation issues):
I have both a V1 and V2 motion sensor in the back of my mailbox. I had to shut off the V2 because it is constantly giving false positive hits. The V1 works most of the time. I’d like to change to a contact sensor, but with the magnet mounted on the mailbox door, I haven’t figured out how to accurately place the sensor half on the inside of the mailbox since, of course, I can’t get at it with the door closed. There must be a clever way, but I’ve not thought of it.