What format is NOT supported for customization of Notifications Sets?

Customization of Notification Sets in the Mimecast system involves the use of specific formats to design and personalize email notifications sent to users. The supported formats for this task include HTML and plain text, which allow for rich formatting and straightforward text messages, respectively.

HTML is particularly useful for creating visually appealing notifications with images, links, and styled text. Plain text, on the other hand, is the simplest format, ensuring compatibility across all email clients without any formatting overhead.

While XML is a versatile and widely used format for data exchange, it is primarily designed for structured data and is not intended for the straightforward presentation of email content. CSV, or Comma-Separated Values, is typically used for data storage and transfer, particularly in spreadsheets, but it does not serve the purpose of crafting notification messages effectively.

C. CSV is the format not supported for customization of Notification Sets, as it's designed for data representation rather than the stylistic and communicative needs of email notifications. Therefore, the correct choice centers on the lack of capability to use CSV as a means for email customization within the Mimecast framework.