Which type of notification does not exist in the Notification Set Definition?

The correct answer is that the Cloud Password Expiring Notification does not exist in the Notification Set Definition. This distinction is significant because the Notification Set Definition is specifically designed to encompass various types of alerts and notifications relevant to system administrators and users within the platform.

Notifications like Server Downtime, New User Registration, and Security Alerts are typical within email security frameworks, as they inform users about critical events that could affect system functionality or security. These notifications help organizations maintain oversight and respond swiftly to any potential issues.

In contrast, a Cloud Password Expiring Notification, while important in general security practices, is not typically listed as part of the standard Notification Set within the context of Mimecast's service offerings. This helps users focus on the more pressing alerts associated with email security and operational integrity without diluting the notification relevance with less critical alerts regarding password management.