So I used that adapter, but I had the problem that the mail server was more important to be accessible without interruption that the ldap server. So I tried replicating the ldap server to the mail server, but since I did not want the password hashes on the mail server, I tried a partial replication which somehow did not always work. Although the ldap-server for the adapter was ldapi:///, it sometimes said 'connection refused' and shunted my mails.
I finally changed LdapMemberships.py to FileMemberships.py, which is minimal and just reads the list entries from files that are created with python's pickle module. The files, on the other hand, are populated by a cron job which contacts the original LDAP-Server. The disadvantage is that changes to the LDAP entries get realized with a delay of one hour at most, but the advantage is that if there is some problem with the LDAP server, the files are just left as they are and the mails get delivered. For my case this is more important.
For the implementation of FileMemberships, I was guided by this module, so thanks for that. If someone else is interested, my version is attached.
** Attachment added: "ldapsync.tar.gz" https://bugs.launchpad.net/mailman/+bug/558106/+attachment/3210464/+files/ld...