Enable group mailing list in Server.app not working

Hello everyone,

I have been trying to enable a new (?) feature of Server.app : "Enable group mailing list". Until now, it does not work for me. When I try to send an email to the group team@pretendco.com, Mail tells me that the smtp server cannot send my message. Here is what I see on the server's /var/log/mail.log file :

Sep 10 16:55:54 testserver01.pretendco.net postfix/smtpd[2311]: NOQUEUE: reject: RCPT from testclient01.pretendco.net[10.1.101.10]: 550 5.1.1 <team@pretendco.net>: Recipient address rejected: User unknown in local recipient table; from=<thomas.baigneres@pretendco.net> to=<team@pretendco.net> proto=ESMTP helo=<testclient01.pretendco.net>

Sep 10 16:55:54 testserver01.pretendco.net postfix/smtpd[2311]: disconnect from testclient01.pretendco.net[10.1.101.10]

It is interesting to note that group mailing list aren't even mentioned in the Mac OS X Server Advanced Administration Guide :

http://help.apple.com/advancedserveradmin/mac/10.8/#apd9430d30c-5796-4a20-a8f9-1 20638938942

Does any has any good/bad experience with group mailing lists under Mountain Lion Server ? Any advice ?

Thanks a lot !