When I send a really basic HTML email via a file using PHP mail it displays perfectly for me. But for my client he sees all the HTML tags in the email.
I have my own dedicated server, to try to problemsolve I ran the exact same file from a Heart Internet shared server I have and the email displayed perfectly to my client (no HTML tags) so it points to something to do with the server.
<?php
$to = '[email protected]';
$subject = 'TOM Testing: HTML displaying correctly?';
$message = '
<html>
<head>
<title>This is a test</title>
</head>
<body>
<p>To see if</p>
<table>
<tr>
<th>You</th><th>See</th><th>The</th><th>HTML</th>
</tr>
<tr>
<td>or</td><td>it</td><td>is</td><td>formatted</td>
</tr>
<tr>
<td>correctrly</td><td>1</td><td>2</td><td>abc</td>
</tr>
</table>
</body>
</html>
';
// To send HTML mail, the Content-type header must be set
$headers = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
// Mail it
mail($to, $subject, $message, $headers);
?>
Here is my php info: http://www.saloponline.co.uk/phpinfo.php