[Dovecot] singe system user for all virtual users

[Wojtek Bogusz]

hi, i would like to have all the emails stored under one system user and 
system group. i keep user passwords in LDAP. for userdb i wanted to use 
static. but i keep getting this error:

Error: Error in configuration file /usr/local/etc/dovecot.conf line 909: 
Unknown setting: userdb
Fatal: Invalid configuration in /usr/local/etc/dovecot.conf

i am using version 1.0.13 my /usr/local/etc/dovecot.conf without 
comments is:

base_dir = /var/run/dovecot/
protocols = imaps imap
listen = *
disable_plaintext_auth = no
shutdown_clients = yes
log_timestamp = "%Y-%m-%d %H:%M:%S "
ssl_disable = no
ssl_cert_file = /etc/apache2/ssl/server.pem
ssl_key_file = /etc/apache2/ssl/server.key
mail_location = maildir:/home/MAIL/%n
mail_privileged_group = mail
protocol imap {
   login_greeting_capability = yes
   imap_client_workarounds = tb-extra-mailbox-sep
}

protocol lda {
   postmaster_address = postmaster at frontlinedefenders.org
   hostname = base
   auth_socket_path = /var/run/dovecot/auth-master
}
auth_verbose = yes
auth_debug = yes
auth_debug_passwords = yes
auth default {
   mechanisms = plain
   passdb ldap {
     args = /etc/dovecot/dovecot-ldap.conf
   }
   userdb static {
     args = uid=mail gid=mail home=/home/MAIL/%n
   }
   auth_userdb =
   user = mail
   socket listen {
      master {
        path = /var/run/dovecot/auth-master
        mode = 0600
        user = mail # User running Dovecot LDA
        group = mail # Or alternatively mode 0660 + LDA user in this group
      }
   }
}
dict {
}
plugin {
}


what do i do wrong?
best regards, Wojtek