Which format is NOT a standard option for customizing Notifications Sets?

The correct answer highlights that PDF is not a standard option for customizing Notification Sets within Mimecast. The customization of notifications typically involves formats that are primarily text-based or designed for easy reading and integration into various email clients.

HTML allows for rich formatting, enabling the inclusion of graphics, colors, and layouts, which can enhance the visual appeal of notifications. Plain text serves a straightforward purpose, ensuring that notifications are readable across all email clients without any formatting issues. Markdown is a lightweight markup language that facilitates simple formatting, making it suitable for notifications that require a touch of style without overwhelming the recipient.

In contrast, PDF is more suited for documents that require a fixed format, such as reports or forms, and is not commonly utilized for dynamic notifications that need to be quickly read or acted upon in an email context. This distinction makes PDF an unsuitable choice for Notification Sets customization in Mimecast.