I want to confirm the details so that I have things straight. You say:
When I enter the SMTP server directly (mail.clubnet.tin.it), it works fine
Does this mean that when you set the value for the mail server within the Universal Email interface that you are entering mail.clubnet.tin.it, or do you mean that when you edit the code and add this in manually that it is working?
I would like to see just the page that has the UE on it to get a better idea for how it looks for you. If you are setting the variable then echoing out the variable it should work all the same. If I have the page in question I might be able to spot an issue or add some debug code to help you get to the bottom of this.