All bounce back emails will now go though an SPF check. If the recipient’s address has a valid SPF record the message will be sent. If the address has an invalid SPF or no SPF record at all, the email will not be sent.
These changes are do to SpamCop’s low tolerance of using third parties to filter bounced mail. The new SPF filtering impacts Auto-Responders and Content Filters with the “bounce” option selected.