It the other email is a POP3 email, you can set up Plus! to check this other email.
To do this, first make sure it is a POP3 email! This kind of email addresses require you to know the POP3 server of that email address to be able to check it.
Then, go to
Plus! > Preferences > POP3 E-mail, and there you fill out the needed data:
email address: that other POP3 email you wanna check
server: the POP3 server for that email
port (the textbox after the server textbox): should 110 in almost all cases
login: the login used for that email
password: the password used for that email
With Plus! in the Italian language, this is:
Plus > Preferenze > POP3 E-mail.
After you've entered the settings, you can push the button
"Test Settings" or in Italian:
"Verificia Cfg".
If everything was filled out correctly you should get a message:
"The server was contacted succesfully" or in Italian:
"Connessione al server riuscita!".
If all this was successfull, Plus! will report the amount of new/unread mails from time to time. And you will also see the current amount of new/unread emails in the top link on your contact list.
Note that Plus! only supports POP3 email addresses!
------
For detailed tutorials about this, see the official help pages:
*
POP3 E-Mail
*
Configuring The POP3 E-Mail Checker
------
PS: Sorry that I can't write all this in Italian, this is the best I can do...
/ 
