Trying to find out cause of blacklisting

Hi:

I've been running mail servers (qmail) for about a decade, and I found 
out yesterday that one of them had been blacklisted by SpamCop.  It was 
automatically delisted in 24 hours per their policy, but nobody from 
SpamCop has replied to my request for details of exactly why it was 
blacklisted.

If I could get a copy of the offending message, with full headers, it 
would be possible to find out how/why this happened, as I don't want it 
to happen again.  The longer the delay, the harder it's going to be to 
find anything in the frequently rotated logs.

I have had no other reports of Spam from that server, so it makes it all 
the more perplexing.  If somebody could point me to somebody who could 
help me on this, I'd appreciate it.

TIA,