Thought about getting a nicer email signature earlier on, while adding the qr code image to my signature, the most natural thing to do was just to upload it to cubie and link it. Just realised that this could result in some interesting phenomenon.
Basically, whenever someone opens the email, it downloads the signature from my server. The server logs would thus be able to serve as some sort of read receipt to track the emails. Assuming I used a unique URL for every email, I would be able to track what time someone opens my email, what ISP/browser he is using etc. without him knowing. Of course, his email client would have to allow images for it to work.
Will try it out for a couple of weeks and observe the results.