You need to configure Roundcube first to get it to work properly with DirectAdmin.
Step 1 – Open Roundcube configuration file located at WEBMAIL_DIR/config/main.inc.php.
Step 2 – Change the line for default folder as below:
$rcmail_config['drafts_mbox'] = <span>'Drafts'</span>;
And change to..
$rcmail_config['drafts_mbox'] = <span>'INBOX.Drafts'</span>;
Step 3 – Repeat step 2 for Spam, Sent and Trash folder.
Step 4 – You need to configure folder display array.
$rcmail_config['default_folders'] = array('INBOX', 'INBOX.Drafts', 'INBOX.Sent', 'INBOX.spam', 'INBOX.Trash');
Step 5 – Enable default folder creation by editing the line below.
$rcmail_config['create_default_folders'] = true;