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-]+)*@".

    // 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.