Why do people still include “Think of the environment, Don’t print this e-mail unless you really have to.” in the thicket of information in their signature block?
I hate it, hate it, hate it!
Why?
Because you are communicating one, two or all of the following messages:
1. I think you are so stupid that you would print e-mails if I didn’t tell you not to.
2. I’m a green paragon, and you couldn’t possibly be as virtuous as me, so let me share my wisdom.
3. Look, this green thing is so easy, all you have to do is add this message to your signature and voilà! Job done.
No self respecting green business would want to patronise and insult their contacts in this way.  So if you are doing it, please stop.