[exim] [off-topic] Using low-priority DNS MX record as spam …

Any thoughts on the idea of dedicating a
host or interface as a trap by giving it
the lowest priority MX record?

I find that the lowest-priority exchangers
in my domains are used primarily (maybe
exclusively, I need to analyze) by
spammers. Could this be a way to detect
spamhosts? Is anyone doing this already?
I realize this could be a colossally
bad idea so I'm looking for thoughts and
the attendant laughter.

-dub