[Exim] keeping tabs on lists

I have set my system up to manage lists along the lines set out in section 42 of
the Exim manual. I would like to see which lists have ceased to be used. The
obvious answer would be to add a dummy user to each list but I don't want to have
to create a user corresponding to each list and then have to periodically empty
all their mailboxes, etc.

Ideally I would like to just do the equivalent of "touch /dirxx/listname" every
time a list is used. Then a simple "ls -lt /dirxx/" to get a list of the names
of the lists in the order of oldest-used-first.

Any ideas on a tidy way of doing this?

Phil.
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Phil Chambers (postmaster@???)
University of Exeter