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;
X-Postfix-Queue-ID: 5BFAB7228206
X-Postfix-Sender: rfc822;
Arrival-Date: Wed, 3 Apr 2013 10:05:53 -0700 (PDT)

Final-Recipient: rfc822;
Original-Recipient: rfc822;
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix;

BUT the message IS delivered to the address which this account is an alias for.

Any ideas?