We do have third-party apps that could be involved, especially mail-chimp, which we use to send newsletters and such. I haven’t been able to find any sign of a confirmation email though. Only the newsletter functions.
The other apps we use seem doubtful to me to be a factor, but here’s a full list: Appstle, Catch Weigth Order Fulfillment, Zapiet, Matrixify, Thinkific, and Collections Import Export Pro.