Email to Mailman list was rejected: User unknown in virtual mailbox table

I have a question concerning Mailman apps that was installed following the Debian 7.0 Perfect Server tutorial.

Information about my server:
Debian 7.0 with Dotdeb, installed based on steps in “Perfect Server” howtoforge.com
The server include postfix, Dovecot, nginx, Mailman
ISPConfig upgraded to version 3.0.5.4p1

My problem:
I sent email to a mailing list address, say [email protected], but the server give this error message:

550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table

What I did to prepare the mailing list:
- Installed Mailman
- ISPConfig3: Email > Domain > Add new Domain (create mydomain.org for email domain)
- ISPConfig3: Sites > click mydomain.org: Add nginx Directives at the Options tab to allow web-access to mailman
- Created first mailing list called “mailman”, and at the same time created mailman admin password
- ISPConfig3: Email > Mailing List > Add new record (create listname mailing list)
- Open web http://mydomain.org/cgi-bin/mailman/admin/listname, login, add Membership

Could anybody explain what should I do to fix this problem?

Regards,
Muhamad

ps. I have checked the contents of the files in /var/lib/mailman/data directory:
aliases contains “STANZA” for mailman (mailman-admin, mailman-bounces, etc.) and for listname
transport-mailman contains STANZA for listname ([email protected] local, etc.)
virtual-mailman contains STANZA for listname. Some lines in the file:

[email protected] listname
[email protected] listname-admin
[email protected] listname-bounces
[email protected] listname-confirm
[email protected] listname-join
[email protected] listname-leave
[email protected] listname-owner
[email protected] listname-request
[email protected] listname-subscribe
[email protected] listname-unsubscribe

 

I checked /var/log/syslog:

May 27 04:38:01 mail postfix/smtpd[24807]: warning: database /var/lib/mailman/data/virtual-mailman.db is older than source file /var/lib/mailman/data/virtual-mailman
May 27 04:38:02 mail postfix/smtpd[24807]: connect from mail-vc0-f169.google.com[209.85.220.169]
May 27 04:38:03 mail postfix/smtpd[24807]: NOQUEUE: reject: RCPT from mail-vc0-f169.google.com[209.85.220.169]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail-vc0-f169.google.com>

Because it says that "virtual-mailman.db is older than source file", I did the following:
cd /var/lib/mailman/data
postmap virtual-mailman

After that, I sent test email to the same list, and I got the following error message:

<[email protected]> (expanded from <[email protected]>): user unknown

Final-Recipient: rfc822; [email protected]
Original-Recipient: rfc822;[email protected]
Action: failed
Status: 5.1.1
Diagnostic-Code: x-unix; user unknown