We have recently been having problems at my organization with being blacklisted. While we have appeared on some of the block lists previously, we are no longer listed yet we are still being blocked by most mail services. We get bounced messages that say undeliverable and others are permanently deferred. I have taken steps to insure that we are not sending out spam, such as running virus scans on all of the computers on our network and setting a rule on our Exchange server that only allows outbound email from an address within our organization. After placing the rule on our Exchange, I set out emailing the postmaster for every mail service that was blocking us in order to be unblocked. It worked for a few days, but now I am seeing that we are beginning to be blocked again. I am unsure of how to proceed, and any input is welcomed.