[Exim] How do I test whether an IP is in a range?

In exim.conf, how do I test whether an IP address is in a certain
range? I want to check whether incoming email is from my local
network, which is all 10.0.x.x addresses, so I'm trying this test:

${if and {{def:sender_host_address} {!eq
{$sender_host_address}{10.0.0.0/16}} } {1}{0}}

But that's returning true for all sender host addresses. Is there a
different syntax I need to use?

And also, where is this covered in the Exim documentation? I want to
know where I missed it... :-)

(FYI, this test is for SA-Exim to determine whether it should run its
spam filter -- I don't want it to run the filter on mail originating
from my local network. SA-Exim uses the same config file options as
Exim itself.)

  ____    |\/|                 Brian Kendig   Set your priorities right.
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