Block nondelivery to internet

Does anybody know of a way to stop Exchange 5.5 from trying to send non-delivery receipts and delivery/read receipts back to an internet address? I’ve recently implemented a new firewall and part of it’s policy to prevent mail relaying is only deliver outbound mail that originates from *@ourdomainname (only from any valid internal user who has our domain name). Exchange as part of being RFC compliant sends delivery and non-delivery notices to SMTP recipients on the internet with the from field null. The firewall then blocks these attempts and the mail piles up in Exchanges outbound queue until they time out. It’s these messages I want to stop at the source.

Thanks
Brian