[Dovecot] ldap uid with static gid problem

[Farkas Levente]

hi,
i'd like to use dovecot to authenticate from ldap. in ldap each user has 
  it's uniq uid and gid. unfortunately with postfix we can't use uniq 
uid/gid since in this case we have ot create all user's home in advance. 
   so we decided to use uniq uid and a common gid in postfix since in 
this case postfix can create new users maildir and deliver new mails. so 
i'd like to use everyone's own uid in dovecot, but a common gid (same as 
in postfix). but how can i do so? i read the wiki, but the problem is if 
i use:
auth_userdb = ldap /etc/dovecot-ldap.conf
then i have to define ldap attrib name in dovecot-ldap.conf:
user_attrs = mail,,,uid,uidNumber,gidNumber
what should i use in the last field in user_attrs?
or it's not possible with dovecot to define this?
i'd be very said in this case:-(
any advice?
thanks in advance.
yours.

-- 
   Levente                               "Si vis pacem para bellum!"