If security is not established properly, employees and others may be able to read sensitive information. Then, instead of being a business tool, email becomes a tool of paranoia. For example, recently a client believed that an employee was reading email communication between the business owners, but could not understand how the employee had access to that mail and could not prove that it was happening. Fortunately, the employee did not cover his tracks well and after a short discussion and visit, I was able to demonstrate the how and identify evidence. While my client was lucky, finding evidence is usually very difficult and expensive.
Digital mail is almost too easy and quick.
The old paper method of write it down and wait till tomorrow before mailing is frequently not considered. And once it's mailed, there is no turning back. Digital mail makes communicating with the masses easy. Therefore, involving some level of corporate affiliation with external use mail may reflect incorrectly or inappropriately as to your corporate opinion. All in all, this can lead to corporate embarrassments.