[Exim] rewriting From: based on To:

Hey folks,

I'm attempting to rewrite the headers of an outbound message based on its
destination. I've set my MUA to use 'eric@???', which is fine for
pretty much everything. However, for local delivery, I would like to have
Exim substitute 'eric@???' for any recipient '@nick.ecks.org'.

Q1007 of the FAQ mentions this, and A1007 gives a rather brief answer.
Without an example, and being fairly new to Exim, following its advice was
a rather daunting task. Here's what I have so far in my rewrite config
section:

eric@??? "${if eq {$header_to:}{@nick.ecks.org} \
{eric@???}fail}"

This, unfortunately, does not seem to work.

Is my head on crooked, is there a single character error there somewhere,
or is this an impossible task? Any help would be greatly appreciated (even
"Stick to snail mail, you freak!")

Thanks!

-- 
"If a system is administered wisely, | eric@???
its users will be content,           | www.ecks.org/~eric
with hacking away at their terminal, | ECSL - What conspiracy?
void of any malicious intent."       | "Tea. Earl Grey. Hot."