mail(）函数存在问题

邮件未发送到收件人电子邮件。请解释下面的代码有什么问题，导致它无法发送。

邮件格式（html）

    <h1>Send Us Your Feedback!</h1>
<form action="send_mail.php" method="post">
<table>
<tr>
<td>Email Adress:</td>
<td>
<input type="text" name="email_from" value="" maxlength="100" />
</td>
</tr>
<tr>
<td>Artist id:</td>
<td>
<textarea rows="10" cols="50" name="artist_id"></textarea>
</td>
</tr>
<tr><td>&nbsp;</td>
<td>
<input type="submit" value="Submit" />
</td>
</tr>
</table>
</form>

发送邮件（php）

    <?php
/*
This first bit sets the email address that you want the form to be submitted to.
You will need to change this value to a valid email address that you can access.
*/
$webmaster_email = "berndamian@gmail.com";
/*
This bit sets the URLs of the supporting pages.
If you change the names of any of the pages, you will need to change the values here.
*/
$feedback_page = "feedback_form.html";
$error_page = "error_message.html";
$thankyou_page = "thank_you.html";
/*
This next bit loads the form field data into variables.
If you add a form field, you will need to add it here.
*/
$message = $_REQUEST['Advertiser email: $email_from 'n Artist id: $artist_id '] ;
$email_from = $_REQUEST['email_from'] ;
$artist_id = $_REQUEST['artist_id'] ;
/*
The following function checks for email injection.
Specifically, it checks for carriage returns - typically used by spammers to inject a CC list.
*/
function isInjected($str) {
    $injections = array('('n+)',
    '('r+)',
    '('t+)',
    '(%0A+)',
    '(%0D+)',
    '(%08+)',
    '(%09+)'
    );
    $inject = join('|', $injections);
    $inject = "/$inject/i";
    if(preg_match($inject,$str)) {
        return true;
    }
    else {
        return false;
    }
}
// If the user tries to access this script directly, redirect them to the feedback form,
if (!isset($_REQUEST['email_from'])) {
header( "Location: $feedback_page" );
}
// If the form fields are empty, redirect to the error page.
elseif (empty($email_from) || empty($artist_id)) {
header( "Location: $error_page" );
}
// If email injection is detected, redirect to the error page.
elseif ( isInjected($email_from) ) {
header( "Location: $error_page" );
}
// If we passed all previous tests, send the email then redirect to the thank you page.
else {
mail( "$webmaster_email", "FEEDBACK FOR AN ARTIST",
  $message, "From: $email_from" );
header( "Location: $thankyou_page" );
}
?>