PHP mail()probably doesn't use smtp to send your emails. I would consider adding the appropriate headers and using a spam counter to help you in your chances, or you can try using smtp through Google's mail servers (although I'm sure this is unlikely).
Here is the spam counter:
http://www.contactology.com/check_mqs.php
Here is an example of some email headers, be sure to include your From header, as its default is probably not suitable for spam filters:
$headers = "From: My site<noreply@my_site.com>\r\n";
$headers .= "Reply-To: info@my_site.com\r\n";
$headers .= "Return-Path: info@my_site.com\r\n";
$headers .= "X-Mailer: Drupal\n";
$headers .= 'MIME-Version: 1.0' . "\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
mail($recipient, $subject, $message, $headers);
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