The tyranny of email

I maintain that programming cannot be done in less than three-hour windows.  It takes three hours to spin up to speed, gather your concentration, shift into "right brain mode", and really focus on a problem. 

Unlike face-to-face conversation and 'phone calls, people can communicate via email without both paying attention at the same time.  You pick the moments at which you pay attention to email.  But many people leave their email client running continuously.  This is the biggest baddest reason why email hurts your productivity.  If you leave your email client running, it means anyone anytime can interrupt what you're doing.  Essentially they pick the moments at which you pay attention.  (Even some random spammer who is sending you a crappy ad for a get-rich scheme.)  This is bad.