We have what is now a long standing system for logging email attachment type information (everyone should have one). For more than a year we've been receiving .iso attachments that caused our program to log cryptic reports claiming that we sniffed these as tar archives that were oddly empty:

attachment application/x-iso9660-image; MIME file ext: .iso; tar no files?!

(This one is unusual in that it had a correct MIME type. The more common MIME type these come with is application/octet-stream.)

Our commercial anti-spam system (Sophos PureMessage) consistently identifies these as CXmail/IsoDl-A.

I've been vaguely wanting to figure out why these messages cause our program to do this and what was actually in these file attachments for some time, but I've been hampered by the fact that I didn't actually have an example file. Our email system consistently rejects these for being malware (and anyway they weren't sent to me), and for various reasons we don't try to have our attachment type logging system save copies of things under any circumstances. I added some extra logging to the system, but it didn't produce anything.

(In some environments, an attachment logging and filtering system would be critical enough that you should be able to capture copies of things that either cause it problems or that seem questionable. In our environment, it's not and making it capture things would raise both operational issues (like managing what it captures and not running out of disk space) and policy ones (around privacy and so on).)

However, I also run a sinkhole SMTP server on another machine. Recently it got a boring spam message which I almost ignored, except that I noticed it had a suspicious attachment that claimed to be an ISO file in the MIME type information (although it had a .img extension). Out of a spirit of curiosity, I extracted the attachment and poked around in it, discovering that it really was an ISO image (well, a UDF filesystem) and contained a single .EXE. Out of more curiosity, I fed it to our attachment logger program to see if it would reproduce the 'tar no files?!' issue. Lo and behold, it did. Now armed with a reproduction case that I could poke around in, I was soon able to narrow this down to a long standing issue in the Python tarfile module.

So, every so often it's useful to get (and capture) spam. Provided that it's interesting and useful spam, at least.