Tips for Dealing with Inappropriate Student Emails?

Hi! Every semester I, like all professors, get rude, inappropriate, whiny, histrionic, and/or barely intelligible emails. Lately, though, it seems like I get more with every class. Next semester I'm adding a detailed "email etiquette" policy to my syllabus, but I wondered if anyone else had suggestions on dealing with the issue? Any approaches you've found effective? Or is it a losing battle, and I should surrender now? My students kvetch when I make corrections on their essays; I can only imagine how they'll take having their emails critiqued! :) Also, I'm only teaching online classes next semester, so I'm a little limited in what I can do. Thanks for any ideas!