Confirmation Email not showing up in GMail

I created an Email form that does two things: populates a mysql db and sends a confirmation email to the person who filled out the form. It works great except for one thing: the confirmation email is not showing up in gmail. When I send it to Outlook accounts or Hotmail and Yahoo, the confirmation arrives without a problem. I'm wondering why is it not working in gmail? Have any of you run into to this problem?

Here is the relevant code:

$editFormAction = $_SERVER['PHP_SELF'];

if (isset($_SERVER['QUERY_STRING'])) {

  $editFormAction .= "?" . htmlentities($_SERVER['QUERY_STRING']);

}

if (function_exists('nukeMagicQuotes')) {

  nukeMagicQuotes();

  }

// process the email

if (array_key_exists('send', $_POST)) {

          $to = $_REQUEST['email'];

          $subject = 'Registration Form';

          // expected fields

          $expected = array('first_name', 'last_name', 'medtech_id', 'job_title', 'company', 'city', 'state', 'email', 'phone', 'contact_me');

          // required fields

          $required = array('first_name', 'last_name', 'job_title', 'company', 'city', 'state', 'email', 'phone');

          // empty array for any missing fields

          $missing = array();

          // assume that there is nothing suspect

  $suspect = false;

  // create a pattern to locate suspect phrases

  $pattern = '/Content-Type:|Bcc:|Cc:/i';

  // function to check for suspect phrases

  function isSuspect($val, $pattern, &$suspect) {

    // if the variable is an array, loop through each element

          // and pass it recursively back to the same function

          if (is_array($val)) {

      foreach ($val as $item) {

              isSuspect($item, $pattern, $suspect);

              }

            }

    else {

      // if one of the suspect phrases is found, set Boolean to true

            if (preg_match($pattern, $val)) {

        $suspect = true;

              }

            }

    }

  // check the $_POST array and any sub-arrays for suspect content

  isSuspect($_POST, $pattern, $suspect);

  if ($suspect) {

    $mailSent = false;

          unset($missing);

          }

  else {

    // process the $_POST variables

    foreach ($_POST as $key => $value) {

      // assign to temporary variable and strip whitespace if not an array

      $temp = is_array($value) ? $value : trim($value);

            // if empty and required, add to $missing array

            if (empty($temp) && in_array($key, $required)) {

              array_push($missing, $key);

              }

            // otherwise, assign to a variable of the same name as $key

            elseif (in_array($key, $expected)) {

              ${$key} = $temp;

              }

            }

          }

            // go ahead only if not suspect and all required fields OK

            if (!$suspect && empty($missing)) {

          // build the message

          $message = 'Sample Message: Hello! You are registered!';

          $message = wordwrap($message, 70);

          $additionalHeaders = 'From: Registration Form on Web Site';

          if (!empty($email)) {

            $additionalHeaders .= "\r\nReply-To: $email";

            }

          $mailSent = mail($to, $subject, $message, $additionalHeaders);

          if ($mailSent) {

                    }

          }

}