take a look in the config file for these options....
// store draft message is this mailbox
// leave blank if draft messages should not be stored
$rcmail_config['drafts_mbox'] = 'Drafts';
// store spam messages in this mailbox
$rcmail_config['junk_mbox'] = 'Junk';
// store sent message is this mailbox
// leave blank if sent messages should not be stored
$rcmail_config['sent_mbox'] = 'Sent';
// move messages to this folder when deleting them
// leave blank if they should be deleted directly
$rcmail_config['trash_mbox'] = 'Trash';
// display these folders separately in the mailbox list.
// these folders will also be displayed with localized names
$rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk', 'Trash');
// automatically create the above listed default folders on login
$rcmail_config['create_default_folders'] = false;
// protect the default folders from renames, deletes, and subscription changes
$rcmail_config['protect_default_folders'] = true;
they let you assign the special folders, weather or not to create them at first login if they dont exist and weather or not to protect them (users cant unsubscribe, delete them etc)