Which blacklist was it and how much did they charge? I have always wondered which are more mafia-esque and which less, as they all act so innocent but are also so adamant about hiding their pricing.
It was backscatterer.org. I don't recall how simple the fix was, but I couldn't find much information on it at the time. The list appeared last in the blacklist tools so I figured it was a fairly new thing.
Alternatively attempt to send a message to a nonexistent address on your MTA using telnet which should throw an error after "RCPT TO" if the server is configured correctly
Steps to test SMTP via telnet: https://my.esecuredata.com/index.php?/knowledgebase/article/...
Thank you for your reply. From my understanding, what you suggest is that a backscatter uses a return path email that does not exists?
My understanding was that a backscatter uses an email that is not his, in order to deliver a message without sending it directly (and making the bounce server act like a spammer).