Which of the following Mimecast Applications can populate Directory Groups?

The correct option encompasses all the provided choices, as each Mimecast application plays a role in the functionality related to populating directory groups.

Mimecast for Outlook allows users to interact with email in a familiar Outlook environment and can utilize directory group information to facilitate email distribution and management effectively. This integration supports a smoother workflow for users who depend on organizational group structures for communication.

Mimecast Server Connection serves as a robust interface between Mimecast services and on-premises email servers. It is designed to enhance directory synchronization, ensuring that user data, including directory group information, is up to date and accurately represented. This is essential for organizations that rely on consistent access control and group-based email functionalities.

The Mimecast Synchronization Engine is specifically tailored to manage directory synchronization tasks. It allows for the matching and updating of user details, including group memberships, across the Mimecast system and the organization's active directory, ensuring all changes in group memberships are reflected promptly.

Together, these applications create a comprehensive ecosystem for managing directory groups and ensuring that email communication is streamlined and efficient. This synergy justifies the encompassing answer that all the Mimecast applications mentioned contribute to the population of directory groups.