Mail opening unexpectedly
After upgrading from El Capitan to Catalina, Mail occasionally opens unexpectedly while I'm doing something else, and sometimes when it's in Sleep Mode behind the screen saver or black screen. It happens maybe once a day or so. It's not a big problem because I just click it off with the red dot, but I'm curious about what's going on, and how to calm it down.
iMac 21.5″, macOS 10.15