Era Eriksson explained to Ken Hooper,
| The w tells Procmail to hang around and wait for the script to
| finish. [Wouldn' you think this ought to be implied by the f already?]
Of course the `f' flag is enough to make procmail wait for the filter to
finish, but the `w' means something more: to wait to learn the exit code of
the filtering command. If sed fails with a syntax error and gives no output,
without `W' or `w' procmail would happily accept the null output as the re-
sults of the filter and go on reading recipes for the now body-less message.
On the other hand, with `W' or `w' sed will respond to a non-zero exit code
by recovering the unfiltered text.