[imp] change sent-mail folder name

[Marco Favero]

Hi all,
I have the following problem.

I have IMP with IMAP. I want to change the name of sent mails folder, by 
default named "sent-mail". I use "Sent" instead "sent-mail".

I tried with this preference in "imp/config/prefs.php":

// sent mail folder
$_prefs['sent_mail_folder'] = array(
    'value' => 'Sent',
// For Exchange server uncomment the line below and delete the line above
//    'value' => 'Sent Items',
    'locked' => false,
    'shared' => false,
    'type' => 'implicit');

...but this change has no effect.
When I compose a mail, a copy continue to move to "sent-mail" folder (if 
"sent-mail" folder doesn't exists it autocreate itself). "Sent" named 
folder instead is not used by IMP.

Software version is webmail 1.0.6. I login with a localized language 
(Italian).

Could you help me?
 
I thank you very much for every hints you could give me.
Bye