The image above is from http://thinkbeforeprinting.org/ — which, amazingly, exists. They — whomever they are — are trying to get lots (everyone?) of people to put that line in their email.
Please, for the love of the Lorax, DO NOT LISTEN TO THEM. You are killing innocent trees.
Let’s say someone prints your email. If you have that line on it, you’re adding at least three lines to the email — the message itself, a horizontal rule above it, and a blank line above that. It could be more than three, but let’s go with three.
Let’s also say that a typical printed page has 50 or so lines. With all the stuff many email clients (e.g. Outlook, below) puts on top of the page, there isn’t a lot of room left for the actual message.
Included quoted text, and there is a very good chance your chiding little message will mean I’m printing an extra page. Which is the exact opposite effect of what you’re after.
And if not? You’re still burning up extra toner and electricity in printing that extra line. Not very environmentally sound. To make matters worse? If, as suggested, you use green text in the message, the person printing the email may default to using a color printer. This uses up even more toner.
Your every-email suggestion is going to have little to no effect on whether I, and for that matter, anyone actually prints an email. So please, think of the environment before having your email ask me to please think of the environment before printing the email. You are doing more harm than good.