I did a search and found a discussion of the same topic here

http://www.ornl.gov/cts/archives/mai.../msg00075.html

Although I had the idea independently, I seem to have come to the same conclusions.

Your IPs should never end up on blacklists, because blacklists only harvest IPs from actual spam sent. As your fake relay should not be doing any actual relaying, it shouldn't end up there.

If worried about bandwidth usage, put some delays in to "tarpit" it, causing the spammer to trickle spam in.

Slarty