Besides the productivity boost I get from my accountability partner, the act of writing out the email (which condenses the scope of my work for that evening into a few sentences) focuses my attention on whatever it is I'm working on, and leaves me with something to chew on in the back of my mind when the afternoon is growing late. By the end of the work day I'm already eager to hit the ground running.
Several key things regarding courtesy and etiquette displayed to email buddies: Firstly, the emails are targeted; I'm emailing people because I genuinely believe they are interested in what I'm working on. These people can be colleagues, fellow hobbyists, and students I've mentored. If I find myself working on a project and have no one that I feel would be interested in it, it's a sign of one of two things: either I need to expand my social network, or perhaps the thing I'm working on isn’t really such a good use of my time.
Secondly, I don't take too much of their time. I expect them to spend maybe 10 minutes of their attention on the correspondence, and if they spend more time than that it's because they were interested enough in what I was doing to spend more time scrutinizing it. (It helps that my projects can usually be presented visually, with a brief series of photographs that require very little effort to digest.) I don't spend every night working on projects, and I usually have 2-3 things that I’m working on at any given time, so each person I maintain contact with gets an average of one email per week.