as per this:
it seems like backscatters - that means that your zcs receive email (ZCS close SMTP connection to the remote MTA server) which is spam (or recipient doesn't exists or any other reason - it will be described in your logs) and after antispam/antivirus check in after-queue ZCS realize that this email shouldn't be delivered, so ZCS will generate DSN and send it to (faked - all or at least most of spams have faked headers) MAIL FROM address.
All of the fields in "Sender" just say "mailer-daemon".
Send logs as already suggested to see details.
Release 7.1.1_GA_3196.RHEL5_64_20110527001604 RHEL5_64 NETWORK edition.