Courier (as of version 0.44.2) has an old hardcoded filter in ldapaliasd.c. This causes operation of ldapaliasd to fail in some setups.
The hardcoded filter is (!(source=*)). The "source" attribute is no longer in Courier's LDAP schema, so this query usually causes a filter decoding error on the side of LDAP server. Some LDAP servers improperly report a success code on this filter, so ldapaliasd works almost fine on them (OpenLDAP 2.1 BDB backend comes to mind).
I've prepared a patch that fixes this bug: courier_ldapaliasd_sourcefield.patch.gz [1]
(update 2004-02-25: this is version 2 of the patch, also fixing the hardcoded filter in virtual domain handling).
(update 2004-03-11: the fix is in the main source repository [2] now).
Links:
[1] http://olo.org.pl/static/admin/courier/courier_ldapaliasd_sourcefield.patch.gz
[2] http://sourceforge.net/project/shownotes.php?release_id=96946