preferences problems

[Gregorio Faraldo]

Hi 
i want that all the mails that the users deleted go to a custom folders by 
default
I configure in the preferences file the name of the folder 
and lock this

$_prefs['trash_folders'] = array (
'value' = > 'papelera' ,
'locked' = > true,
'shared'= > false,
'type' = > 'implicit'
);



When the user log in for firts time and delete a message i get the error
The folder "papelera" was not created. This is what the server said: Invalid 
mailbox name
There was an error moving messages to the trash. This is what the server 
said: Error in IMAP command received by server.

What i can do ?
Many Thanks