This is a great question. It’s essential to note that the drone can’t fly in the rain unless they are waterproof or watertight. All drones have extremely sensitive technological components than can shut down if any raindrops touch them. Of course, heavy damage to these components might not happen immediately, but over time, for sure. DJI Matrice 300 RTK is waterproof and can fly in rain.  Moreover, it has a protection rating of IP45. However, this rating is not permanent and it can decrease if there is wear and tear. On the official DJI website, you can find information that the drone should not fly in rain heavier than 100mm/24h. So, my suggestion here is to always check how heavy is the rain actually even if your drone is waterproof and watertight.