On email in general: When you send an important document or message via email, do you ever get left wondering if the recipient ever actually received it? You assume so. But wouldn't it be great if that recipient shot back an email as soon as they received that important document or message - just to confirm.
After all, when you send important information via an overnight postal service, it is protocol to have the recipient sign for that document once received.
As a rule, whenever you receive an important document or message via email, reply to confirm its receipt. If you are really short on time, just click "reply" and write, "received."
If you reply to confirm to a sender who is a prospect, don't forget to include your signature and tag line, perhaps use this opportunity to add value your reputation.
If you reply to confirm to a sender who is a customer or member, don't forget to include a thank you for recent business or membership.
Every message sent from your business email is a form of business communication. Remember your etiquette, mind your p's and q's, and be professional.
More on email and Internet etiquette: http://www.library.yale.edu/training/netiquette/index.html