Convert PHP Email Code to Using SMTP Authentication

One of my contact forms no longer is able to send (it used to until some server updates were made), for it keeps posting the following error:

Warning:  mail() [function.mail]: SMTP server response: 501 5.1.7 Invalid address in C:\contact.php on line 71

This form does work on three other servers, so obviously the form is in working order but not on this particular server.

I was recommended to "set the PHP script up to log in to       the SMTP server" using this example:

<?php
// set flag to indicate whether mail has been sent
$mailSent = false;
if (array_key_exists('eList', $_POST)) {
    // mail processing script
    // remove escape characters from POST array
    if (get_magic_quotes_gpc()) {
    function stripslashes_deep($value) {
    $value = is_array($value) ? array_map('stripslashes_deep', $value) : stripslashes($value);
    return $value;
    }
      $_POST = array_map('stripslashes_deep', $_POST);
      }
 $email = $_POST['email'];
        // check for valid email address
    $pattern = '/^[^@]+@[^\s\r\n\'";,@%]+$/';
    if (!preg_match($pattern, trim($email))) {
    $error['email'] = 'Please enter a valid email address';
    }
    
// validate the input, beginning with name
    $name = trim($_POST['name']);
    if (empty($name)) {
        $error['name'] = 'Please enter your First Name';
        }
    $lname = trim($_POST['lname']);
    if (empty($lname)) {
        $error['lname'] = 'Please enter your Last Name';
        }
    $department = $_POST['department'];
    if ($_POST['department'] == '') {
    $error['department'] = 'Please select a Department';
    }
    
//     check the content of the text area
        $messageBody = trim($_POST['message']);
        if (empty($messageBody)) {
        $error['message'] = 'Please enter your message';
        }
        
    // initialize variables
    if (!empty($_POST['url'])) {
    $to = 'email@email.com';//
    $subject = 'Suspected as SPAM';
    $host = 'smptout.serverserver.net';
    $username = 'username';
    $password = 'password';
    } else {
    $to = 'email@email.com';//
    $subject = 'Here we are';
    $host = 'smptout.serverserver.net';
    $username = 'username';
    $password = 'password';
    }
    
        $SpamErrorMessage = "No URLs permitted";
    if (preg_match("/http/i", "$name")) {echo "$SpamErrorMessage"; exit();}
    if (preg_match("/http/i", "$lname")) {echo "$SpamErrorMessage"; exit();} 
    if (preg_match("/http/i", "$email")) {echo "$SpamErrorMessage"; exit();}
    if (preg_match("/http/i", "$messageBody")) {echo "$SpamErrorMessage"; exit();} // check for spam
    //build the message
    $smtp = Mail::factory('smtp',
                                                                                            array('host' => $host,
                                                                                                                    'auth' => true,
                                                                                                                    'username' => $username,
                                                                                                                    'password' => $password));
    $message = "To: $department\r\n\r\n";
    $message .= "From: $name $lname\r\n";
    $message .= "$email\r\n\r\n";
    $message .= "Question/Comment: $messageBody";
    
    //build the additional headers
    $additionalHeaders = "From: Contact <here@here.com>\r\n";
    $additionalHeaders .= "Reply-To: $email";
    
//send the email if there are not errors
    if (!isset($error)) {
    $mailSent = mail($to, $subject, $message, $additionalHeaders);
    // check that the mail was sent successfully
    if (!$mailSent) {
        $error['notSent'] = 'Sorry, there was a problem sending your mail. Please try later.';
        }
    }
}
?>

Thank you for your help!