[Exim] wildcards and relay_domains

I have an exim system with 15,000 + domains listed in a CDB file for
use in relay control. The problem comes in that we want to allow
subdomains of those domains to be relayed also, but it seems that exim
won't do that without an explicit listing in the CDB file. Putting a
wildcard subdomain in the CDB file doesn't work (i.e. `*.domain.com'),
and the relaying is still rejected.

Is there any way around this? It would be good if exim could
recursively remove subdomains until it either runs out of string, or
finds a match in the relay list. With CDB file formats, its not even
that much of an overhead.

Thanks,

Gary