I just got some Zigbee contact sensors. I might use one for the mailbox.
It uses a CR2450 battery.
I'm wondering: how long would the battery would last in an upstate NY winter?
(PS: It even shows temperature)
In my experience, that's going to have more to do with how hard that sensor has to work to connect to the mesh than with its temperature.
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I have Iris v2 Motion/Temperature Sensors in two freezers, and they have lasted over a year and are still going.
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Are they Zigbee, and what size battery do they have?
iris2 has CR2 battery
mine is in freezer 1.5 year
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It’s zigbee and uses a CR2 battery.
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I just did a quick search and saw something like 486mah for the CR2450 and 800mah for the CR2, so about twice the capacity. Mine might not make it too far. I still have to figure out how to tape it to the plastic mailbox in the cold temps-amazing how it won't work. Maybe I'll try putting one in the refrigerator as a torture test. I just ordered 5 of the Visonic MCT-350's for 5 bucks each. I had previously received 6 for, like, 10 bucks each. Should've done a little more research, I guess, but the price was enticing.
This stuff works well for sensors outside:
https://www.amazon.com/Scotch-051141397252-Extremely-Mounting-414P-ST/dp/B018GKCI82
Just cut a sliver to fit the sensor base.
As it is now cold, warm up the plastic mailbox and the double-sided stuff a bit with a hairdryer just before applying it, or bring the mailbox inside to install the sensor. But once applied, these never come off - I use them for all sorts of purposes at even my place in the white mountains of NH, where it gets very cold.
I'd have to bring a generator out to the end of the road, and that's not gonna happen.
3M has a complete array of tape. I have a roll of their VHB that I bought for sticking a dashcam to a windshield, but I think there are different VHB products as well.
I've come up with a new plan: buy a new mailbox, work on it inside the house. The big reflective numbers are not cheap though.
I use a smarthings multi-sensor for my mailbox, which uses a CR2450. It it three floors down and maybe get down to 15c and its still going.
But from the temperature side i have the same sensor in my fridge at around 2c and it at first dropped quickly to 75% abut has been okay for about 3 months.
I tried the same in the freezer it dropped to 15% and only lasted a few days, but started to work again once warmed up.
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I'm a little surprised the signal makes it through the sheet metal of the fridge.
I just installed it. Four screws to take off the mailbox. A lot easier doing most of the work inside and the tape looks like it stuck. Sensor seems like it works, but the temperature is reacting slowly. It's about 28F outside.
Next step is announcing it somehow. I get pushover notices on my phone, but I'd have to get my wife involved too, I guess. We're not into the smart speaker scene. Maybe a chime of some sort.
I suspect my fridge hasn't got much sheet metal, if any, covering the bottom. I've noticed my Zigbee contact sensors (CR2 powered Iris V2's, which can go almost two years between battery replacements) are child devices of a Zigbee repeater located on the floor below.
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You can probably add a larger , more temperature suitable battery anyway if you think ‘inside the box’ 
For cold weather application, instead of using tape you could use a drop of silicone caulk.
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Why not just add the wife to pushover and send notifications to her phone too? Could go the route of announcing through a smart speaker if that’s more desirable.
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I didn't cover the sensor with plastic or anything. That could be a problem, even though it's mostly sheltered in the box.
edit: I put some clear packing tape over it.
Yes, that's the way I'm going to go.
Great idea, hadn't thought of that.
The magnet fell off, luckily in the mailbox and not the snow. 
.
I put some of that black Gorilla duct tape on it and the sensor as well.
It's good camouflage as well.
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Plus, I know that it's 26.6F out there by the road!
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