The FROM address in the invitation e-mail is always being set to the webserver system user address

The problem is that in the call to drupal_mail inside the rsvp_mail function, $from is always being set to NULL.

function rsvp_mail($mailkey, $to, $subject, $body, $from = NULL, $headers = array()) {
  $site_name = variable_get('site_name', 'drupal');
  $title .= 'RSVP: '. $subject .' ['. $site_name.']';
  return drupal_mail($mailkey, $to, $title, $body, $from = NULL, $headers);
}

To fix it the NULL should be removed from the call to drupal_mail:

function rsvp_mail($mailkey, $to, $subject, $body, $from = NULL, $headers = array()) {
  $site_name = variable_get('site_name', 'drupal');
  $title .= 'RSVP: '. $subject .' ['. $site_name.']';
  return drupal_mail($mailkey, $to, $title, $body, $from, $headers);
}