I've run the abuse.net open relay test and it comes back negative (actually the test fails, I think because of firewall settings). Some of our IPs have been blacklisted, but it is difficult to tell which IP is blocked. We 'own' the first fifty or so IPs in our subnet and, for example, roadrunner has from time to time blocked the whole subnet.
Just for clarification... it's not considering fishy if an email for client.com comes from example.com?
Many times were able to find out we're blocked, but it is just a pain to hunt down two of these every month (or whatever the frequency is).
Thanks for you input.