[Exim] database lookups

I know Exim supports SQL databases (when compiled in), but I'm confused
about how to use that functionality. For example, I discovered I was
somehow open to spam relaying, so I ran a "lockdown" on exim.conf --
everything from helo_verify to never_users=mail [!]. I figured that
later, after I was removed from the RBL, I could start backing down the
crazier measures.

The first thing I discovered was that many hosts would fail the
helo_verify test. Initially, I started adding manually verified hosts to
the config file, but the list became longer than was reasonable.
Currently, I've disabled the test, because we were refusing too much mail
because of it. But what I'd like to know is, assuming I have the link
set up already, how would I rewrite the "helo_verify" command to look up,
in a database, hosts that do not require verification?

Thanks...