So this is weird and I'm not quite sure how to resolve it...
We had a user, but then deleted it and instead made it an alias for another account.
However now when you send it a message it bounces:
Reporting-MTA: dns; mail.ourdomain.com
X-Postfix-Sender: rfc822; email@example.com
Arrival-Date: Wed, 3 Apr 2013 10:05:53 -0700 (PDT)
Final-Recipient: rfc822; firstname.lastname@example.org
Diagnostic-Code: X-Postfix; x.com
BUT the message IS delivered to the address which this account is an alias for.