[Exim] local_domains lookup deferred

I am experiencing a problem with a newly deployed Exim server. It is
configured similarly to our existing server, which works just fine.
Admittedly, I am not an LDAP guru, and I believe the problems to be
LDAP related.

When I send mail to more than one recipient, only the first gets
delivered. The others get permanently recycled with 'local_domains
lookup deferred' messages. The other addresses will deliver
successfully if entered alone on a To: line or entered as the first
recipient. This is true whether the additional recipients are entered
as a comma seperated list on the to: line or as CC: or BCC: recipients.

My local domains ldap line looks like this:

local_domains =
"ldap;ldap::///LDAPUSERS?cn?one?(&(cn=$key)(|(mx=*)(cname=*)))"

the LDAPUSERS macro looks like this:

LDAPROOT = dc=virtual,dc=helixsystems,dc=com
LDAPUSERS = ou=Users,LDAPROOT

our LDAP schema contains mx attributes, which dictate which server
speaks for the domain, cname attributes which give mailhost expansion
(i.e. poopoo.com has an mx attribute pointing to mailhost.poopoo.com)
the cn contains the domain to be looked up (poopoo.com.) Only the
mailhost.poopoo.com entry would contain an mx attribute, conversely
only the poopoo.com entry would contain a cname.

This is Exim 3.35, from Debian package, running on Debian 3.0R1.

I'm sure there's a reasonable explanation for this, but I've been
looking at it too long I think. Any ideas?

Thanx,
Clint