[Exim] An unusual question

I have Exim 3x configured as a outbound-only relay on my router/firewall
(headless)pc. In the event of network outages or more likely, operator
incompetence, my outbound mail piles up and eventually gets frozen. I would
like to work out a mechanism to beep the pc speaker on delivery failure, so
I can investigate the problem. I'm looking for some sort of hook that can
call an external script on delivery failures. I see the delay_warning and
error_* directives, but I can't seem to find exactly what I'm looking for.
Suggestions?