The most important task before implementing any associated email controls to prevent sensitive information from being emailed outside the organization by employees is to develop an information classification scheme.
An information classification scheme is a framework that defines the categories and levels of sensitivity for different types of information, such as public, internal, confidential, or secret. An information classification scheme can help implement email controls by providing criteria and guidelines for identifying, labeling, handling, and protecting sensitive information in email attachments. The other options are not as important as developing an information classification scheme, as they do not address the root cause of the problem or provide the same benefits. Requiring all employees to sign nondisclosure agreements (NDAs) is a legal control that can help deter or penalize employees from disclosing sensitive information, but it does not prevent them from emailing it outside the organization. Developing an acceptable use policy for end-user computing (EUC) is a governance control that can help define and communicate the rules and expectations for using IT resources, such as email, but it does not prevent employees from emailing sensitive information outside the organization. Providing notification to employees about possible email monitoring is a transparency control that can help inform and warn employees about the potential consequences of emailing sensitive information outside the organization, but it does not prevent them from doing so. References: CISA Review Manual (Digital Version), Chapter 5, Section 5.3.2