Hi Tomdkat,
welcome to the forums!
Your interpretation is correct. The email was sent by a server where the From header was your domain, but the RFC 5321 Mail From (most commonly known as the retun-path) was a different domain. This kind of report is typical of forwarded mail, especially if DKIM passed, and was your domain.
Forwarding in this case would be automated forwarding, like with a mailbox rule, or whole mailbox forward. In those instances, headers are maintained (such as the From and DKIM header) but the return-path will typically be rewritten through a process called Sender Rewriting Scheme. This is just an example however and may no apply here, but since this IP is not recognized, the fact DKIM passes means it had to have been sent from an authorized source originally.
I hope this helps!
