There’s one guy in your outfit who understands the need not to write stupid e-mails: That’s the guy who just spent all day in deposition being tortured with the stupid e-mails that he wrote three years ago.
That guy will control himself. He’ll write fewer and more carefully phrased e-mails for the next couple of weeks. Then he’ll go back to writing stupid stuff again, just like everyone else.
You can’t win this game; no matter what you say, people will revert to informality and write troublesome e-mails. But you’re not allowed to give up. What’s an in-house lawyer to do?