Custom Double Opt-In Email

Hello, I have the same problem described in this other thread , but I can’t understand the solution proposed there. Basically the problem is that checking the Double Opt-in option sends the user an email to confirm his subscription. By default the email configured in the first Mailster form is sent. But I need to be able to change the confirmation email (double opt-in) for some Gravity forms. But I don’t see how to change the email content (of the double opt-in) according to the gravity forms used. How can I do it? Thanks in advanced.