Quick post with my humble opinion on how to do email right. Here's a simple rule that I was taught once and has done me well ever since:
If you get an email and intend to respond, do so within 24 hours (during the work-week).
This does not mean you have to send the full response if you don't have time. Something like - got your message; I'm slammed for the next two days. I'll get back to you by Wednesday night - is totally fine.
That way the sender knows that the message has been received and isn't left hanging waiting for a response.