[exim] Small feature request - fallback_hosts

There might be a better way to do this but I don't see it. I am trying
to send spam to various blacklists providers for information harvesting.
The feed is rather high volume and sometimes the receiving end can't
keep up. This is email that doesn't have to be delivered so I want to
make only one delivery attempt and if it doesn't work, discard it. No
message queueing or retry.

What I thought might be a good way to do this is that if you specify a
fallback_host = 0.0.0.0 it tells the system to discard if delivery
fails. Of course if there's a better way that would be great.