Any system that is accessible through the Internet is subject to attempts to infiltrate the operating system's security or to exploit security weaknesses of services running on the system. Administrators should be concerned about not only these kinds of blatant attacks but also inadvertent actions by users. Some of the most serious breaches of security are caused by users whose actions result in the release of sensitive data or cause a flood of mail from the Internet.
If you are using Internet Mail Service, you should consider how to minimize the security risks when you connect Microsoft Exchange Server to the Internet. You can configure Microsoft Exchange Server and Windows NT Server so that people in your organization can send and receive Internet mail but unauthorized users can not access your system from the Internet. You can also set up other mechanisms such as firewalls that protect against attacks.