Which of the following is NOT a custom report type available?

The choice of Email Routing as the option that is not a custom report type available is accurate since Mimecast offers various reporting functionalities, but Email Routing typically refers to the process of directing emails through different paths rather than a distinct report category focused on statistical analysis or metrics.

Custom report types generally emphasize quantifiable data that can assist in analytics and decision-making. For instance, Email Volume and Email Bandwidth provide insights into the amount of email traffic and the data size being transferred, respectively. Email Statistics typically encompass a broad range of metrics that help users monitor and analyze email activity.

In contrast, Email Routing primarily involves the technical configuration of how emails are processed and directed within the Mimecast service, thus not aligning with the notion of generating standalone custom reports that present analytical data. Therefore, identifying Email Routing as not being a custom report type highlights an understanding of the distinction between routing functionalities and report-driven insights within the Mimecast platform.