spamc should probably have some minimal header-rewriting capability for the case
where a failure occurs.

Something like, "X-Spam-Status: Unknown, error="Connection refused"" would
certainly cut down on the messages to the mailing list saying, "SA isn't
processing my mail!"

This would require reading in headers separately, which means more complexity in
the spamc code. That's not desirable because of the security implications, but I
don't see another reasonable way....

The good thing is that you don't have to "parse headers" per se. You just have
to find the end of the headers and tack the status on to the end in the event of
spamd failure.