Hi there, @Plume_Blanche 
I see that you’ve already had a chat with my colleague Daria. As she said, you can enable the autoresponder option in the Notifications tab:
And you can edit the text of the success message in the Edit Texts section on the Settings tab:
However, there is no option to edit the full confirmation email and we have a request for this on the Wishlist. Feel free to upvote this idea - Add an option to edit confirmation email