The solution I have pulled out of GTD is that once a day I clear everything out of my inbox, and anything that needs a reply gets put into a todo list. Slightly later in the day, I groom that Todo list, and work out which items I will today, and set aside 30m in which to do those. If there's an item in there that will take a significant investment in time -- including a certain email -- then I'll make an appointment on my calendar to do it.

I'll respond to interesting email that comes in during the day if I feel like it / if I'm bored, but knowing that once a day I do a complete clear-out is helpful to me