mail problem

[mshen]

I write a simple program like this.

<?php
mail("[e-mail]", "My Subject", "Line 1");
?>

I believe no syntax error for this,but I get a error message that list here.I do not know why.Please help me .Thanks

Warning: Unknown error in d:\program files\apache group\apache\htdocs\test\mailtest21.php on line 2

I use Appach Server in windows 2000 with PHP 4.

Thanks