This has been bothering me for literally a year. I can't seem to get rid of these relentless emails from Microsoft Teams. If I want to see random water cooler chatter I would ask for it. Finally, the latest email from them tells you exactly how to shut it off...

Update your email preferences in Teams which way buried in Profile picture > Settings > Notifications > Email > Missed activity emails > Never

So that is it, click on the upper left and you will see the Settings for Notifications and like the very annoying feature in Slack (which has all notifications turned on), you have to go to every team and do the same thing over and over.