SMTP error code 550 not seen as bounce

From the collection of uncaught bounces I'm dealing with it seems
that Mailman does not treat an unknown user account as a fatal
error (despite the 5xx code). This must be a bug. It's a real
headache as these clearly fatal errors constitute more than 80% of
the unchaught bounces I must wade through. Here are several
examples (and I have many more if you need them!):

From: <email address hidden>
Date: March 25, 2004 11:06:51 PM EST
To: <email address hidden>
Subject: Returned Mail - Error During Delivery

------ Failed Recipients ------

<email address hidden>: Requested action not taken:
mailbox unavailable. [SMTP Error Code 550]
-------- Returned Mail --------

.....

From: "InterScan MSS Notification" <email address hidden>
Date: March 25, 2004 11:08:25 PM EST
To: <email address hidden>
Subject: Mail could not be delivered

This message was AUTO GENERATED. Please DO NOT RESPOND to
this EMAIL.

Sent <<< RCPT TO:<email address hidden>
Received >>> 550 5.1.1 <email address hidden>... user unknown

Unable to deliver message to <email address hidden> (and other
recipients in the same domain).

.....

From: "InterScan MSS Notification" <email address hidden>
Date: March 25, 2004 11:08:25 PM EST
To: <email address hidden>
Subject: Mail could not be delivered

This message was AUTO GENERATED. Please DO NOT RESPOND to
this EMAIL.

Sent <<< RCPT TO:<email address hidden>
Received >>> 550 5.1.1 <email address hidden>... user
unknown

Unable to deliver message to <email address hidden> (and
other recipients in the same domain).

[http://sourceforge.net/tracker/index.php?func=detail&aid=925319&group_id=103&atid=100103]