[exim] rewrite smtp-time rcpt

our lowest MX preference is a spam trap
its set up as a second IP on our primaryMX and a rule defers all mail
addressed to it...

what im looking to do now though is rewrite the rcpt address at SMTP
time so that all mail aimed at the spamMX gets routed into a mailbox on
that machine > spam@mailhub4 for baysian input / browsing / all sorts of
fun stuff

anybody