SMTP error code 550 not seen as bounce

[Cashwell]

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]

[Mark Sapiro (msapiro)]

This has nothing to do with the 550 per se. What it has to do with is heuristic bounce recognitions ability to recognize these (non-standard?) DSNs.

There have been many changes to bounce recognition since 2004, but it is not possible to say whether these specific bounces would be recognized without seeing all the message headers and MIME sub-part headers if any.