I have the same issue with Microsoft Teams. Someone once invited me to join a group on Teams, and now (years later) I get occasional but persistent emails from Teams. I can't stop them, because the setting apparently needs me to be signed in, but I don't have an account. I mark them as spam, because it's all I can do.
I think I experienced the same quirk - basically, you need a Teams license to sign in to teams (in order to access an outside organization as a guest), so if you don’t pay for it via your AAD org (or via a free live.com account) you’ll get emails while not being able to accept the invite or access the channels.
