Itzy Sabo, an email management blogger, has written an article on the responsiveness of your email may affects how people think of your competence. He takes couple of examples to demostrate this point:
… If people’s opinion of you were based solely on your email responsiveness, can you imagine what they would think of you?
This works both ways, as the above examples show. It’s so easy to be responsive, and if you don’t have time to deal with an issue, be honest about it — to yourself and the other party…
How can Responsiveness help your Professional Reputation? – [Email Overloaded]