Hello,
Just after having posted my message, I tested again, and this time the message was not deleted but simply flagged.
Maybe I didn't logged out and in again after changing the config ?
Or because I changed this :
// old $rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk');
$rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk', 'Trash');
Sorry if I was wrong, I'll post here again if the problem confirms. Currently everything seems correct again...