[Mailman-Users] mailman problem with exim

[Lorenzo Coronati]

hi all
I'm trying to make mailman work with exim (on a centos server) but I'm 
facing a problem:
when a subscriber posts a message, it isn't forwarded to the other 
subscribers, and no error message is sent back to the sender
this is what I find in the /var/log/exim/main.log:

2011-07-28 09:01:50 1QmKbO-0004h7-NS <= lorenzo at coronati.com 
H=adsl-ull-193-191.49-151.net24.it ([192.168.0.207]) [151.49.191.193] 
P=esmtpa A=plain_login:lorenzo at coronati.com S=644 
id=4E310960.6050509 at coronati.com T="provo io"
2011-07-28 09:01:50 1QmKbO-0004h7-NS Failed to find group "" from 
expanded string "${lookup mysql{select users.gid from users,domains 
where localpart = '${quote_mysql:$local_part}' and domain = 
'${quote_mysql:$domain}' and users.domain_id = domains.domain_id}}" for 
the address_pipe transport
2011-07-28 09:01:50 1QmKbO-0004h7-NS == |/usr/local/mailman/mail/mailman 
post mailinglistaips <mailinglistaips at aips.it> R=system_aliases 
T=address_pipe defer (-28): Failed to find group "" from expanded string 
"${lookup mysql{select users.gid from users,domains where localpart = 
'${quote_mysql:$local_part}' and domain = '${quote_mysql:$domain}' and 
users.domain_id = domains.domain_id}}" for the address_pipe transport

what I understand is that exim (which is configured to manage the email 
addresses stored in a mysql database) tries to expand the mailing list 
looking for subscribers via sql, instead of letting mailman to do this
am I right?
and then, what do I have to do to fix this?

thanks in advance
lorenzo