As you may have heard, Hillary Clinton has faced some questions recently regarding her use of a personal email for work purposes. Read more here.
What's the concern about this? It seems that, particularly in a public service job, there is some worry that her state-department related emails could remain private instead of being recoverable by the government. Generally, emails sent from a work address are accessible by the employer. Employers are thus able to monitor what their employees are sending and receiving on the job. Some people are concerned that Ms. Clinton's emails cannot be monitored by the government, and by extension, the taxpayers.
Employers have the right to review emails sent from their email addresses and computers, for many reasons. These reasons include making sure employees aren't sending things that embarrass the company from company email addresses and ensuring that employees are actually working during work hours. Further, employers may wish to access email communications later to document what was said to or by the company, for example in the case of a contract dispute.
Employers can require employees to use their work emails for work business for these reasons.