[Mailman-Users] Bouncing a user repeatedly.

[P.I.Julius]

Hi,

I have a little problem with my mailman setup, and I would like to ask you
guys for an opinion about it. I have a few lists set up on my mailman, but
I just noticed that for one of my lists there is something wired going on.
The server keeps bouncing a user which is an unknown user, so I have a lot
of unwanted emails going on on my server. For eg. this is one of them:

Mar 28 17:50:48 lists postfix/smtpd[26762]: connect from
localhost.localdomain[127.0.0.1]
Mar 28 17:50:48 lists postfix/smtpd[26762]: 3DCF928002:
client=localhost.localdomain[127.0.0.1]
Mar 28 17:50:48 lists postfix/smtpd[26762]: 3DCF928002: reject: RCPT from
localhost.localdomain[127.0.0.1]: 450
<cva34.internetdsl.tpnet.pl at lists.hndomain.org>: User unknown in local
recipient table; from=<fk-bounces at lists.hndomain.org>
to=<cva34.internetdsl.tpnet.pl at lists.hndomain.org> proto=ESMTP
helo=<80-70-88-201-BusName-dc.hfc.ccs.net>
Mar 28 17:50:49 lists postfix/smtpd[26762]: disconnect from
localhost.localdomain[127.0.0.1]

As you can see the bounce is sent out from fk-bounces at lists.hndomain.org
to cva34.internetdsl.tpnet.pl at lists.hndomain.org but we don't have such an
user on our server, so why do we get all this emails sent out? it happens
every seconds, so it really uses a lot of cpu resources.

Could someone please help me with this?
Thanks in advance!
Julius