Researcher Jakob Nielsen recently released an excellent report on how email newsletters are used successfully, including ethnographic observations on how professionals relate to their inbox. Nielsen believes that good newsletters respect and support their audience, making it easier to manage information.
I recommend the entire executive summary, but the bottom line was summarized in the Wall St. Journal excerpt below.

WSJ.com: Talking Tech, Interview of Jakob Nielsen by Lee Gomes, 2006-Jun-20

You say you like traditional newsletters emailed to customers
instead of news feeds. Why so? Don’t they contribute just as much to
information overload?

The best newsletters really drive customer relationships. But they have to be very good, very targeted to a specific user’s actual needs. And it’s got to be very timely as well. For example, one newsletter is called ‘Your Baby This Week,’ and is targeted at new parents and, in particular, new mothers. …people really look forward to getting that particular newsletter. Unless a newsletter is very good, people will just say, ‘Oh no, more information.’