All of these situations were legal disasters for the people involved. Any e-mail message can be subpoenaed and used in litigation against the manager, the company, or the employee. Once electronic files are subpoenaed, they must be preserved. 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.