The ZixGateway will remove any attachments from emails sent by the Postmaster or Mailer-Daemon accounts. These addresses are typically used to send bounce (delivery failure, or NDR) notices back to the original sender.
The attachment on a bounce notice will usually include the original email. Since the ZixGateway does not encrypt "bounces", attachments are removed to ensure no sensitive content (such as the original email) is transmitted in an unencrypted form.
The body of the email will remain intact, which includes the exact cause of the delivery failure.