[John]
I was just going through the posts in this mailing list, and have come upon (so far) 4 of these weird posts. [...]
I'm getting such as well.
Is this some kind of spam?
This is indirect. Here is a possible scenario. You sent email to a first someone, at some time in the past. That first someone's machine runs Windows and recently got infected by a computer virus. The virus found your email address into this first Windows machine, and used it to forge the `From:' header of infecting emails broadcasted from the Windows machine to various people, one of which also run Windows and also got infected. But this time, the ISP of this second guy has been aware that he was sending spam (because he was infected as well) and terminated his account. When a message from the first someone was sent to the second someone, it was intercepted by the ISP, and because of the forged From, that ISP sent a message to you explaining that the message from the first Windows machine would not be delivered to the second Windows machine.
If it is, it's the strangest I've ever seen/encountered,
Don't stop breathing. You'll surely see worse. :-) -- François Pinard http://www.iro.umontreal.ca/~pinard