Unable to send test mail in Notification

namespace OMV;

/**
 * @ingroup api
 */
class Email {
    private $from;
    private $to;
    private $subject;
    private $message;

    /**
     * Constructor.
     * @param from The sender email address.
     * @param to The receiver email address(es), e.g. 'test@xzy.com' or
     *   'xzy@123.org,test@xzy.com'. Use the character , or ; as seperator.
     *   Additionally this can be an array of email adresses.
     * @param subject The subject.
     * @param message The message.
     */
    public function __construct($from, $to, $subject, $message) {
        list($this->from, $this->to, $this->subject,
          $this->message) = func_get_args();
        // Modify the receiver list.
        if (TRUE === is_array($this->to))
            $this->$to = implode(',', $this->to);
        else
            // Prepare the receivers of the email. The formatting of this
            // string must comply with RFC 2822.
            $this->to = strtr($this->to, [ ';' => ',' ]);
    }

    /**
     * Send the email.
     * @return Return TRUE if successful, otherwise FALSE.
     */
    public function send() {
        // Prepare additional email headers.
        $headers = [];
        // Set date in RFC 2882 format.
        $headers[] = sprintf('Date: %s', date('r'));
        // Append the sender address.
        if (!empty($this->from))
            $headers[] = sprintf('From: %s', $this->from);
        return mail($this->to, $this->subject, $this->message,
            implode(PHP_EOL, $headers));
    }

    /**
     * Send the email.
     * @return void
     * @throw \OMV\AssertException
     */
    public function assertSend() {
        if (FALSE === $this->send()) {
            throw new \OMV\AssertException(
              "Failed to send the email. Please check the logs.");
        }
    }
}