[Mimedefang] delete spam based on score

[Lucas Albers]

> Put this in sub filter_end, in your mimedefang-filter file.  (Note that we
> discard at a score of 20)
>
> Ken

I discard at 9.1 but with an important caveat.
I do an action_bounce on mail I reject.
This makes absolutelly sure a valid message never disappears.
The sender is always notified that their messages has been rejected.
I've rejected thousands of messages and only had less than a handful of
false positives.

If for whatever reason a valid message is rejected, if the sender is
informed then they will be able to resend their message, or contact me to
fix it.

Management gets really pissed when messages just blackhole.

Using action_bounce on messages does not generate any extra overhead.

==luke