Risks of using HTML Mail and HTTP proxy "censorware" together

(As published in Risks Digest Volume 21: Issue 5)

Summary: Unseen things in HTML mail may trigger HTTP censorware.

First, the data points:

1. Many workplaces, including mine, have HTML-"enabled" mail software on the desktop.
2. Many workplaces (though not as many), including mine, make use of HTTP proxy "censorware" to catch employees trying to access "bad" sites (porn, hate sites, hacking sites, etc).
3. Those sites, like many others, tend to use 1x1 GIFs for spacing and the like.
4. Users who read HTML mail rarely view the source.

Now, the risk: