The maildrop line in postfix originally looked like this:

maildrop unix - n n - - pipe flags=DRhu user=mail argv=/usr/bin/maildrop -d ${recipient}

and the authldaprc: ... LDAP_MAIL mail LDAP_HOMEDIR mailMessageStore LDAP_MAILDIR mailMessageStore LDAP_GLOBAL_UID mail LDAP_GLOBAL_GID mail LDAP_DEREF never ...

Now I changed the authldaprc file by setting the LDAP_UID to uidNumber and the LDAP_GID accordingly and removed the '-d' switch from the postfix master.cf file