Marc has posted [a great idea for email productivity](http://blogs.officezealot.com/ marc/archive/2005/02/10/4109.aspx). It uses mnemonics in the subject line to mark what response is required from a message, eg. “NRN - Whatever subject” to designate “No Reply Needed”.
I often get a lot of emails with a replied comment or a ‘thanks’ when I didn’t need or want one. It creates noise at both ends.