You can set the columns to show by default in the config file:
// These cols are shown in the message list. Available cols are:
// subject, from, to, fromto, cc, replyto, date, size, status, flag, attachment, 'priority'
$config['list_cols'] = array('subject', 'status', 'fromto', 'date', 'size', 'flag', 'attachment');
Also users can change their personal settings for the columns by clicking on the gear icon on the left size of the headers.