A few days ago, I started encountering spam messages which cannot be processed due to what appears to be a problem with resolving an ipv6 address.
Here is the tracking link: https://www.spamcop.net/sc?id=z6437392727zd5176b494aaf328f9c8ad3ba8a7727ebz
The error I received with this particular one was:
No unique hostname found for source: 2002:a17:902:aa4a:0:0:0:0
Possible forgery. Supposed receiving system not associated with any of your mailhosts
Will not trust this Received line.
Mailhost configuration problem, identified internal IP as source
The receiving account is a gmail account, and the sending IP (according to gmail) is 188.8.131.52, which belongs to ocn.ad.jp aka the ISP from Hell. There are also numerous references to ocn throughout the header, so I am confident that they are the source.
The only place that the aforementioned ipv6 address occurs in the entire header is a single X-Received: line. Every gmail message I've checked has an X-Received: line (invariably with a 10.xxx.xxx.xxx IP address), so I don't know if it's a google error, or if ocn spammers have figured out how to spoof this field, or if the problem is internal to Spamcop. I couldn't find anybody having a similar problem in the forum. I even tried going through the spamcop registration process again, but that didn't solve the problem.
The only thing that seems consistent is that these errors only occur with spams sent from ocn.
Hopefully, somebody out there has some ideas.