Mark,
You have choices that will affect how much e-mail people get, and they also can make choices about how much they want to receive.
First of all, within their profile, their e-mail can be turned on or off. If it is off, they will receive no e-mail from the site.
Secondly, there is a News Forum in the top of every moodle to which everyone is automatically subscribed. This can be hidden if you don't want it, but it is useful for sending messages to the entire group for many things. Then, as editing teacher, when you add forums, you will set them either as "Everyone is subscribed" or "Everyone can choose to be subscribed". If you choose to subscribe everyone, then everyone will receive every e-mail. If you choose the latter, then they will choose to receive the forum messages that interest them.
Hope this helps! Moodle on!