The `[[sender.first_name]]` will be populated with whoever the "default" sender of the email is configured as. If you change the From person right before clicking Send, the variable will not update and the email tagline will still be sent the way you visually see it in the email editor.  We'd like to make this a little more dynamic at some point soon (including ability to customize your own signature lines), but for now the only solution is to manually modify the email template before you click send.