Core_Email::_send_with_mail

protected function _send_with_mail()
{
    if ($this->_safe_mode === true)
    {
        return mail($this->_recipients, $this->_subject, $this->_finalbody, $this->_header_str);
    }
    else
    {
        // most documentation of sendmail using the "-f" flag lacks a space after it, however
        // we've encountered servers that seem to require it to be in place.
        return mail($this->_recipients, $this->_subject, $this->_finalbody, $this->_header_str, '-f '.$this->clean_email($this->_headers['From']));
    }
}