Hi Riccardo,
Glad to hear almost everything is now working the way you want it to. Unfortunately, due to the age of my students, school policies, and the way I had to set my Moodle server up, I never used any Moodle email capabilities, so I can´t reproduce that part to know what string to search for.
Could that possibly be your site email no reply entry. Site administration > Server > Email > Outgoing mail configuration.