There is no valid reason to keep ESI longer than necessary for a) business purposes and b) legal or regulatory requirements. Implementing an automatic delete policy (60 days to 365 days) is a sound business practice. Email is a communication tool--not a business records repository in most businesses.

In addition to the cost of storage and archiving software (read the blog as an advertisement) you must consider that if litigation occurs someone (read lawyers as several hundred $$ per hour) are going to have to sift through all of that email to determine whether it is relevant to the lawsuit (or worse, the DOJ investigation).

All of those emails and IMs that would have been telephone calls in the past ("Want to go to lunch, now." "Sorry, can't make it yet." Etc.) are clogging up your servers and costing you dearly when the eventual lawsuit gets filed.