Life of a one man IT department found a good article by Kim Komando titled, "The e-mail you save can be held against you." Great stuff to pass around the office. I have actually come across problems where people have kept so much old email, that their email clients barely runs. I usually force them to start deleting items immediately. Kim makes some good points like forcing yourself to decide if an email is a company record or not.
You're not worried. But then you get a subpoena, ordering you to submit any e-mail from the past three years that bears on the case. There's nothing in the e-mail that would affect the case, and Screwup knows it. But you'll have to dig up everything you have, and your lawyer will go through it, looking for relevant material. How many hours, at $200 per hour, will it take her to do that? So you swallow hard and give Screwup $15,000 to go away. You may never have thought about this, but you can bet that the plaintiffs' bar has.