When a cross-appointee in the Math Department sends a message, everyone gets it. I was interviewing someone recently from a very large bank. The e-mail prerogative only goes from the CEO down three levels. These can be as complex as discussing the composition of the student body, the value of different forms of research, the coordination of course outlines, or strategies for dealing with budget cuts. Anyone can raise an issue, and members of the Faculty can reply to the individual or to the entire community.
Delete keys are the antidote to bloated e-mail boxes.