[Exim] lookups with explicit keys

Is it possible to do a lookup on something besides $local_part in a DBM
file? E.g., using $domain in the user = cdb;/.. directive? The nisplus
example isn't quite enough (for me) to extrapolate something simpler.

http://www.exim.org/exim-html-3.20/doc/html/spec_6.html#SEC142 scares
me. This is one part of exim I would love to see overhauled and
simplified.

Thanks,
Paul

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