[Plugin Contact Form 7] After submission nothing happens

Hi there,
I just installed CF 7 on my 3.0 and tested it whit std built-in email form.
What it does when I press Submit is to show the Ajax loader gif and waiting for ever (it’s still there now…).
Besides, I can’t find no place to set SMTP.
How can I solve the problem?
Thanks 1k
Riccardo

https://www.ads-software.com/extend/plugins/contact-form-7/