To view the logs of "Thank You" emails sent from your survey, go to the Notifications page and click the "Logs" icon located at the bottom right corner. This will display the complete email log.
You can refer to this screenshot for guidance: https://app.screencast.com/06yxtmDidE4Gu
Can custom variable mapping be set up globally, for all current and future surveys? Use case: customer is using the same custom variables for all surveys.