PHP Email Validation

<?php
function check_email($email) {
    $err = '';

    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        return 'Invalid email format';
    }

    list($alias, $host) = explode("@", $email);

    if (!checkdnsrr($host, "MX")) {
        return 'No MX record/invalid domain';
    }

    getmxrr($host, $mxhosts);
    $connected = false;

    foreach ($mxhosts as $mxhost) {
        $socket = @fsockopen($mxhost, 25, $errno, $errstr, 10);
        if ($socket) {
            $connected = true;
            break;
        }
    }

    if (!$connected) {
        return 'Cannot connect to email server';
    }

    $out = fgets($socket, 1024);
    if (!preg_match("/^220/", $out)) {
        fclose($socket);
        return 'No response from server';
    }

    fwrite($socket, "HELO $host\r\n");
    fgets($socket, 1024);

    fwrite($socket, "MAIL FROM: <{$email}>\r\n");
    $from = fgets($socket, 1024);

    fwrite($socket, "RCPT TO: <{$email}>\r\n");
    $to = fgets($socket, 1024);

    fwrite($socket, "QUIT\r\n");
    fclose($socket);

    if (!preg_match("/^250/", $from) || !preg_match("/^250/", $to)) {
        return 'Server rejected address';
    }

    return $err;
}