My mail server got blacklisted because of backscatter. That is where the sender sends spam to an address that doesn't exist on your server, but they also forge the "from" address, so the "this message could not be delivered" response, complete with the spam message, is sent "back" to the forged address (the spammers real target)

It took several months or a large payment to the people maintaining the blacklist for my server to get unblocked (it wasnt critical so I didnt pay). Seemed like extortion to me, but Google and others respected this particular blacklist.

Im not sure if this list still exists. It was on the blacklist checking websites at the time