我的sendmail.php无效。不发送电子邮件

Question

<?php 
$to = "jerrelldc@gmail.com";
$subject = "This is subject";
$message = "This is simple text message.";
$header = "From:".$_POST['from']." \r\n";
$retval = mail ($to,$subject,$message,$header);
if( $retval == true )  
{
    echo "Message sent successfully...";
}
else
{
    echo "Message could not be sent...";
}
?>

为什么不发送电子邮件？我错过了什么吗？