Thanks for your answer. Yes, the path I moved (and renamed) email-alternative.php in is indeed /wp-content/extentions/ (as stated in /wp-content/plugins/newsletter/subscription/email.php:6).
Even when I directly modify email.php in the plugin/subscription folder, the mail stays the same (even when I leave it completely blank).
When I press “resend confirmation”, I now get my modified mail, because I edited page.php (it doesn’t load images though), but when I delete and re-enter my email address on the front-end (I used the shortcode), it just sends the blueish mail (also even when I leave email.php both blank).
I hope I gave you enough information!