Not Sent Tracking Extract is the tool that should provide a list of subscribers who didn't receive the expected emails. Not Sent Tracking Extract is a type of tracking extract that allows you to export data about subscribers who were excluded from an email send due to various reasons, such as suppression lists, publication lists, frequency capping, etc. You can use this tool to identify why some subscribers didn't receive your emails and take corrective actions if needed. _sent Data View is a system data view that allows you to query data about subscribers who were sent an email within the last six months. 'Subscribers Not Sent To' report is not a valid report in Marketing Cloud.