Have you ever noticed that e-commerce receipts and online reservations tend to come from an email address in the form of "noreply@domain.com", followed by a message at the bottom of the email reminding you not to reply to that email? There's such an easy fix to this that it's embarrassing the customer service industry hasn't caught on.
Here's one that I got from Enterprise Car Share, a ZipCar competitor (red highlights mine; personal information removed):
