Ok thank you very much, I will try to do as you suggest later. 

Riccardo

@Robert, 

Sorry but I don't understand: how can be possible that I have problem with the wp_mail function but if I create a new account in a forum, I receive a registration confirmation email? Does this use mail function as well?

Thank you for your explanation 

Yes, it's possible. You have some plugin or there is a SMTP solution / configuration which works for WordPress somehow (using some alternative hooks), but when other plugins call wp_mail() WordPress function it can't send. wpForo calls wp_mail() function which is based on PHP mail() function. There is some hack from other solutions in one of these level, in PHP or in WordPrdpress.