Contact Form 7 sometimes doesn’t work

My Contact Form 7 in some period of time doesn’t work in other works fine.

I am getting message:
Failed to send your message. Please try later or contact the administrator by another method

I tried to debug this and found that CF7 will call wp_mail.

It will call this code from Contact Form 7:

if ( $send ) {
    return @wp_mail( $recipient, $subject, $body, $headers, $attachments );
}

But this will return false.

Hase anyone some idea what can be problem.

EDIT:

When it call wp_mail it will throw error in this part of wordpress code:

if (!$this->smtp->data($header . $body)) {
   throw new phpmailerException($this->lang('data_not_accepted'), self::STOP_CRITICAL);
}

EDIT:

This data function will at the first line will call function “sendComand”:

public function data($msg_data)
{
    if (!$this->sendCommand('DATA', 'DATA', 354)) {
        return false;
    }
    .....

It will failm in this if statment.

Here is sendCommand function:

protected function sendCommand($command, $commandstring, $expect)
{
    if (!$this->connected()) {
        $this->error = array(
            "error" => "Called $command without being connected"
        );
        return false;
    }
    $this->client_send($commandstring . self::CRLF);

    $reply = $this->get_lines();
    $code = substr($reply, 0, 3);

    if ($this->do_debug >= 2) {
        $this->edebug('SMTP -> FROM SERVER:' . $reply);
    }

    if (!in_array($code, (array)$expect)) {
        $this->last_reply = null;
        $this->error = array(
            "error" => "$command command failed",
            "smtp_code" => $code,
            "detail" => substr($reply, 4)
        );
        if ($this->do_debug >= 1) {
            $this->edebug(
                'SMTP -> ERROR: ' . $this->error['error'] . ': ' . $reply
            );
        }
        return false;
    }.......

and this will fail in this last if statment, this condition in if (!in_array($code, (array)$expect)) will be true.

So $expect was provided on sendCommand function call:

$this->sendCommand('DATA', 'DATA', 354)
(array)$expect) == Array([0] => 354)

and $code we will get from get_lines() function:

 $reply = $this->get_lines();
 $code = substr($reply, 0, 3);

When it fail in $reply is next value:

 $reply = 550 5.4.5 Daily sending quota exceeded. u1sm14669850qat.27 - gsmtp

And here is get_lines() source:

https://github.com/WordPress/WordPress/blob/master/wp-includes/class-smtp.php#L820