PHP Email validation function

My PHP email validation function nowadays is:

function isEmail($value, $network_validation = true) {

    // Create the syntactical validation regular expression
    // (broken in 2 lines for better readability)
    $regexp = "/^([_a-z0-9-]+)(\.[_a-z0-9-]+)*@".
              "([a-z0-9-]+)(\.[a-z0-9-]+)*(\.[a-z]{2,4})$/i";

    // Validate the syntax
    if(preg_match($regexp, $value)) {
        if (! $network_validation)
            return true;
   
        $tmp = explode('@', $value);
        $username  = $tmp[0];
        $domaintld = $tmp[1];
 
        // Validate the domain
        if (getmxrr($domaintld, $mxrecords) || checkdnsrr($domaintld, 'A'))
            return true;
    }
    return false;
}

This is for my own reference, maybe this is all messed up, so use it at your own risk.