[exim] Running a hook on successful delivery

We use exim to accept our customers' outgoing mail, deliver it to a
driver=pipe transport that transmogrifies the message, and then resubmit
it for delivery. All of that works.

But I want to know when an email has left the system. If there is no
other way I'll just grep out the timestamp from the logfile in a
long-lived perl process. I'm hoping there's a more exim.conf-ey way to
log successful delivery - something like what acl_smtp_data_250 might be
if it existed.

Any ideas?