'Add Access rules of mail.message for non-employee user:
- read:
- raise if the type is comment and subtype NULL (internal note)'

@api.multi
def check_access_rule(self, operation):

    if self.user_has_groups('base.group_public'):
        self.env.cr.execute(('SELECT    id    FROM    "%s"    WHERE    website_published    IS    FALSE    AND    id    =    ANY    (%%s)' % self._table), (self.ids,))
        if self.env.cr.fetchall():
            raise AccessError((_('The    requested    operation    cannot    be    completed    due    to    security    restrictions.    Please    contact    your    system    administrator.\n\n(Document    type:    %s,    Operation:    %s)') % (self._description, operation)))
    return super(MailMessage, self).check_access_rule(operation=operation)
