Have you ever tried creating a rules page like this? It’s only relevant to situations where you’re putting your email address in a public place and welcoming perfect strangers to email you.
I notice Rachel Andrew has a page that redirects people to different places before giving out her email.

On the contact form on CSS-Tricks, I make note that we can’t do direct one-on-one technical support.

That makes me feel better at the form-letter response I give emails that come in asking for that.
Maybe that’s the least these kind of warnings and pages can do. If they don’t actually reduce load, they reduce guilt of not responding to emails that didn’t bother to read it or for sending a canned response.
Anyone out there have a rules page of sorts about being emailed?