I finished my message with legitimate questions and an offer to help them with whatever problem they were having if they were willing to provide details. I've never sent any type of nasty email message to a customer nor potential customer.

If someone writes in overly aggressive or just shooting off their mouth, I'll also usually wait a day before replying to make sure I don't write anything off the cuff. Generally the person writes back with an apology.

In some other business types I wouldn't be so kind, but with a web-based business you have to be.