Destroying evidence can lead to court penalties and sanctions and may severely limit the options of the company at trial. It is very easy for experts to discover that electronic files have been purged. Most companies maintain back-up tapes often going back several years. Even if back-ups are not available, experts look for suspicious patterns. Not that deleting files will help you.
Experts can reconstruct electronic files that have been deleted and overwritten three times.
The FBI can read files overwritten five times. Personal information may inadvertently be disclosed to third parties without authorization. All this subjects the sender and the company to liability. Don't collect, store and file hoards of e-mail messages just because you can. Understand that everything in e-mail can be reached by subpoena.