[Plugin: Gurken Subscribe to Comments] Notifications for invalid email addresses

We have a problem with the notifications that are send when someone entered an invalid email address – or rather: those mails are sent to the wrong address themselves, since we now use a different one as the “Sent”-address.

I already changed the mail address field in the plugin and also in the mysql-table newsletter_users. I am not sure why some mails are still returned to the old address. Are those old subscriptions/mail notifications being delivered in the name of the old address? Or did I overlook a field in the mysql-table?