Not really all that critical IME and not the issue that the OP is facing. In his case the MTA (Message Transfer Agent) is rejecting the connection based strictly on the IP address of the sending system, not based on the content of the message.

To the OP, if you are using an ISP as your mail relay agent then you'll likely need to get them to contact the blacklisting agency. The hard part may be determining which service has you blacklisted. If the rejection note doesn't include it you may have to go to the IT staff of the intended recipient to figure out what service they are using.

Or, if you are sending directly from your own connection (i.e. email server in your network, not a relay through their server) then you need to get your home/office IP address cleared.