[exim] Deleting outbound mail from 1 user

We're testing a mailing list package, and I've been asked to temporarily
delete all the messages from the list. Basically they would like exim
to tell them everything worked, and then pipe the email to /dev/null
instead of actually sending it.

Is there a way to get exim to do this? I've been looking over the
documentation, and I figured it out for inbound mail, but not outbound.
Suggestions on how to do this, or where to look are much appreciated.

Aaron