when using a remote checkout method, you cannot send the email from the confirm page when the confirm page posts, the email must be sent from the checkout success page instead.
This is because the form on the confirm page is set to post directly to the remote payment gateway server.
You will need to add a set session value server behavior to the confirm page. Set the trigger to any form post, the name to "EmailAddress". For the value, click the lightning bolt, and select the Email address form element.
now add the universal email behavior to the checkout success page, use the EmailAddress session variable for the from address and set the trigger to before page load.