Earlier quoted context omitted.
> keep in mind that relay IPs, and perhaps even subnets, may show up on various blacklists Have you got an example of that? I know a few relays intimately and I've never seen this.
It happened to me when I ran a tor relay (not an exit) on the same ip as a mailserver. There is one rbl that automatically adds you if you do this. Mind you, I've never found a mailprovider using this list (can't remember the name).
Can you see if you can find the RBL in question? Since there are a _lot_ of relays around the world, surely someone else must have noticed?