Ok I changed the user mailman in master.cf...
now i'm greeted with another error:
Jul 15 15:36:37 SWH-iRed-1 postfix/qmgr[10540]: 34DB340CCA: from=<test@sensusplenoir.com>, size=592, nrcpt=1 (queue active)
Jul 15 15:36:37 SWH-iRed-1 roundcube: User test@sensusplenoir.com [209.208.0.193]; Message for mailman-join@sensusplenoir.com; 250: 2.0.0 Ok: queued as 34DB340CCA
Jul 15 15:36:37 SWH-iRed-1 postfix/pipe[10650]: 34DB340CCA: to=<mailman-join@lists.sensusplenoir.com>, relay=mailman, delay=0.46, delays=0.29/0.01/0/0.16, dsn=5.3.0, status=bounced (Command died with status 1: "/usr/lib/mailman/bin/postfix-to-mailman.py". Command output: TO ACCESS THE MAILING LIST SYSTEM: Start your web browser on http://lists.sensusplenoir.com/ That web page will help you subscribe or unsubscribe, and will give you directions on how to post to each mailing list. )
Jul 15 15:36:37 SWH-iRed-1 postfix/cleanup[10643]: 9C44A40CD0: message-id=<20130715203637.9C44A40CD0@SWH-iRed-1.sensusplenoir.com>
Jul 15 15:36:37 SWH-iRed-1 postfix/bounce[10654]: 34DB340CCA: sender non-delivery notification: 9C44A40CD0
Jul 15 15:36:37 SWH-iRed-1 postfix/qmgr[10540]: 9C44A40CD0: from=<>, size=3045, nrcpt=1 (queue active)
Jul 15 15:36:37 SWH-iRed-1 postfix/qmgr[10540]: 34DB340CCA: removed
Jul 15 15:36:38 SWH-iRed-1 postfix/smtpd[10554]: disconnect from SWH-iRed-1.sensusplenoir.com[127.0.0.1]
Jul 15 15:36:38 SWH-iRed-1 postfix/pipe[10657]: 9C44A40CD0: to=<test@sensusplenoir.com>, relay=dovecot, delay=0.81, delays=0.18/0/0/0.63, dsn=2.0.0, status=sent (delivered via dovecot service)