After posting to a newsgroup for the first time, "unknown user" bounce returned

I tried posting to a sourceforge.net list
(gmane.comp.finance.aqbanking.devel) for the first time, 
I recieved the usual message requesting a reply to establish 
an account.  After that, I got:

----%<---------------------------------------------------------------
Mail delivery failed: returning message to sender
>From : 	Mail Delivery System <Mailer-Daemon <at> plane.gmane.org>
Date : 	Mon, 16 Jul 2012 17:25:26 +0000

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

aqbanking-devel <at> lists.sourceforge.net
SMTP error from remote mail server after RCPT TO::
host mx.sourceforge.net [216.34.181.68]: 550 unknown user

------ This is a copy of the message, including all the headers. ------

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