The ease of forwarding messages and using mailing lists means that employees may inadvertently send sensitive information outside of the company. Email can be sent without a lot of reflection. Most employees don't really think of the fact that when you send an email there will be a copy on your computer, the network backup tape, etc. It's frequently harder to get rid of email than any other written document. If a company is sued, it is routine for the other party to ask the company to produce all their records, including email.
Email is a really juicy target because it can be searched by keyword.
There's no reason not to routinely delete email. However, an email-deletion system must preserve any documents that the company is legally required to keep. Pat Gleason, general counsel in the Florida attorney general's office says their policy is to print out relevant email and file them with related paperwork. That way, if we get a public records request for all the email written five years ago our office can produce it. The system seems cumbersome, and Gleason says someday it will probably be done electronically.