Which of the following elements is used to set all policies in Mimecast?

The correct answer revolves around the concept of "Date validity." In Mimecast, policies are often configured with specific time frames to determine when they are active. This means that the effectiveness of these policies can be controlled based on particular dates, allowing administrators to schedule when a policy should begin or end its applicability.

Using date validity helps in implementing time-bound rules efficiently, such as seasonal policies or temporary bans, thus providing flexibility and precision in policy management. Allowing policies to be effective only during certain periods ensures that they adapt to changing organizational requirements or compliance regulations over time.

The other options, while relevant to certain aspects of policy management, do not directly define the overarching mechanism for setting all policies in Mimecast. For example, policy override pertains to the preference hierarchy of multiple policies, bi-directionality focuses on the directionality of communications, and source IP control deals with network-related configurations. These elements play important roles but are not the primary means of establishing policy parameters like date validity is.