Like he says though, you have to be careful with this. I once had a client who would constantly email me with support questions, many of which I answered multiple times but I guess it was easier for him to just email me instead of searching his email logs. I would always answer back right away, which would mean he would write me back with another question, and the cycle would repeat itself indefinitely. It basically became instant messaging with a 20 minute delay, and totally killed my productivity while making me hate this person.
Also, I try to be as honest with my billing as possible, which means that if each support question takes 5 minutes out of my day, I might only bill this person for 30 minutes, even though the true cost to me is much greater because of my lost productivity.
My solution was to ask him to either send me support questions in batch unless they were real emergencies, or pay me to set up a bug/feature tracker.