[fetchmail] Flusing exim queue after running fetchmail

I use fetchmail and exim on my desktop machine, and would like to have
fetchmail, running in daemon mode, run `exim -qff' (flushing the queue)
after doing a download. Reading the manpage, I gather that you could do
something like that with `postconnect'. I don't get how to actually
implement it, however, so if anyone would like to give me an example,
it would be very nice.