Also not mentioned is monitoring if your users actually read the emails. A web bug will work (some of the time) as will checks to see if users ever respond (eg if there is a link to click for full details) or login to the site after email receipt.

LinkedIn noticed that I never read one of the group messages I was getting and so switched to a far more infrequent digest of highlights. I think they may even have unsubcribed me completely from one group.