[Exim] ldap aliases

I am having some trouble finding the right solution to setting up user
aliases in virtual domains.

In the ACL, I verify aliases recipients like so......
${lookup ldapm {user="cn=admin,dc=localsurface,dc=com" \
pass=PASS ldap:///ou=${quote_ldap:$domain},dc=localsurface,\
dc=com?mailAlias?sub?}}

(which looks to me like an expensive look up)

But then this email fails all conditions in subsequent routers so
bounces. In my ldap directory structure, mailAlias is an attribute
string that is the local part of the email address alias. How then can
this address be routed properly?

I was thinking address rewriting, but how? I also thought about ldap
symbolic link but found no good documentation about that.

BTW, I did implement aliases another way that worked where the aliases
were not attributes but were their own RDNs. That looks good on paper
until you get 500 users with 2 aliases each and quickly it's
unmanageable. The aliases need to be attributes to the cn where all of
the other user specific data is kept AND needs to be able to handle
multiple returns, I guess using ldapm lookup.

Any help deeply appreciated,
Thanks,
Craig