I spend many working days away from the office, and many office hours in meetings. Like most people in large organizations, I receive hundreds of actionable emails a week. I can only access these while at work. And my working days are usually over-committed.
Consequently, I just cannot give every email equal attention. Email is, of course, a broken tool - and a working culture that expects a rapid, pavlovian response from the recipient is not one in which anyone can concentrate long enough to do high-quality work.
To manage this as best I can, I use a kind of 'triage' system, based on the book 'Getting Things Done': each email is automatically 'read' and categorized, and I re-prioritize and work through these lists and my voicemail a couple of times a day. Incidentally, this means that receiving a read receipt for an email does not mean that I've read it!
So, if I haven't responded to your email, I'm either not in, or I'm focussing on delivering work required urgently by another customer. Sorry, but there it is.
And if I've sent you a very curt email - say, four sentences long - I'm doing my bit to save you time (see http://four.sentenc.es/). Feel free to do the same to me.