PHP web contact form getting "stuck" in spam filter

For years, a PHP web contact form I made for someone was working just fine, but now it keeps getting stuck in the e-mail hosting portion of that site. I get the web form mail just fine in my Gmail, but he wants to use his Outlook mail and the hosting company cannot figure out the problem. The weird thing is, I’ll call about the issue and they’ll get it working for a few days before it just stops again.
I cannot get my old client to switch to another hosting company. Is there anything I can do to make the form not get stuck?

This is the PHP code, which does work fine in getting to my G-mail:

<?php

$todayis = date(“l, F j, Y, g:i a”) ;

$recipient = “webmaster@xxx.com”;

$subject = ‘Market Analysis Request’;

foreach($POST as $k=>$v){
if(!empty($v) and $k <> ‘button’ )
$post .= ‘<b>’.str_replace($k,'
‘, ’ ‘).’: </b>’. htmlentities($v) .’ <br />';
}

//$post = print_r($_POST,1);
$content = “Content-type: text/html; charset=iso-8859-1”;

$message = " $todayis [CST] <br />

". $post;

mail($recipient, $subject, $message, $content) or die(“Error!”);

?>