[Exim] Setting up Exim for multiple users

I have Exim 3.33 working fine alongside UW imapd for me as a single user
running under MacOSX (Darwin). But I am having problems figuring out how
best to configure Exim to work for multiple users. I run Exim primarily as
an smtp server accepting remote mail for local users, appending it to a
local user file that is accessible to imapd. Fairly standard, I would
imagine.

My mailbox is located in /Users/cb/Mail/mbox (cb is my username)
My email address is chris@???. The default system_aliases director
correctly resolves chris to cb from /etc/aliases. For single user mode I
have set user = cb in the system_aliases director. All well and good.

But what should I set 'user =" to in the system_aliases director to accept
mail for any local user? Or is there another solution?

I have tried a variety of permutations. A logical one (I would have thought)
would be to set "user = $local_part" but that didn't work: for example with
mail to kath@??? (username = kb) I get the following debug
messages:

file lookup required for kath
in /etc/aliases
lookup yielded: kb
LOG: 0 MAIN PANIC
Failed to find user "kath" from expanded string "$local_part" from the
system_aliases director

because kath is an alias for user kb

I suspect I'm missing something simple and obvious (and apologise for
wasting your time if I have), but all help gratefully received.

C
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