I ran into one of these in the very early 00s; was working at a university (back in the days when a couple of people would run all the central servers, running Linux on beige PCs.) We had some anti-spam/AV software that looked at every incoming email hooked into Postfix, and the server kept running out of disk space.

Eventually tracked it down to an email which contained a zip of stock trading data – just the three letter stock code and the shift. It wasn't malicious, it just had an extraordinarily high compression ratio!