mail(); -> 500 internal server error

[Самурай]

<?
is (!isset($form))
{
echo "<form method=POST action=$PHP_SELF>
Nickname <input type=text name=nick><br>
Email <input type=text name=email><br>
<input type=text value=0 name=form>
<input type=submit value=send>
</form>";
} else {
$message = "$_POST[nick] entered email address $_POST[email]";
mail ("feedback@creative.net.ua", "feedback submission", $message);
}
?>

тести...