It perplexes me that so many people lack e-mail etiquette. People who rely on e-mail for primary communication for business. They should know better.
I am usually careful about composing business or formal e-mails. Much can be (or not be) conveyed through text. I try to cushion messages with gratitude (without seeming too effusive).
It doesn't take that much time to write "I really appreciate your asking; however..." instead of a curt and ambiguous "no." Curt replies really rub me the wrong way, even if they were not intended to be abrasive. That's bad for business.
Oh, and, never returning my e-mails? You're just digging a hole.