Since the mass mailer is not working, I tried to insert an opt-in form in Doplhin 7.3.2 from SendInBlue.
I still have a site running on 7.1 and there it works fine when I add the HTML code in a block on the homepage.
For some reason this same code doesn't work on the 7.3.2 version (EVO template).
I'm always being redirected to a separate page to fill in the form, where in the 7.1 version it immediately sends the email address and gives a 'success' reply in the same block.
Where could the problem be?
The code I generated from the SendInBlue site is attached.