really no good way to know 100% without seeing the full real headers, but if they have the actual IP that your domain reverses too, you need to dig into your mail logs and make sure your not sending out spam!!!
1st of all make sure your email server is not open relay.
Email Server Test - Online SMTP diagnostics tool - MxToolbox you can plug in your mail servers IP or name and it will check to see if it is open for relay.
2nd if it's not, then dig into your mail server logs and do a search for
[email protected] which appears to be the email address that the error was generated for. If it's in your mail log files you have an issue somewhere, could be a script or an open port that is allowing people to funnel mail thru your server.
If it's not in your mail logs, chances are they are just spoofing your email address and there is really nothing that you can do about that
If you do find that that email address was sent mail by your server, my recommendation would be to stop mail services until you can get it fixed! Yes you will loose mail, but the flip side of that coin is, every piece of mail that they send out of your box makes you that much more likely to be blacklisted, then your looking at months of spotty mail service till you can get all the majors to un-blacklist you.
hope this helps!