The 'tracking' is done by logging the IP address of the machine/server that sent the request for the image. The initial request will be your email program, if it is set to display images/html. The image, itself, is probably a simple but transparent gif (1 x 1 pixel is as small as possible) that will never be seen by the naked (or even clothed) eye. The 'logging' simply confirms that your addy was real and that you actually opened the message. SPAMmers appreciate the cooperation provide by opening a message
with html enabled. That way, they can add your addy to their ever growing list of 'valuable' addresses that can be sold or used for further 'correspondence' to you. Of course, many politicians are busy a lot of the time. Seeing all those lobbyists takes a lot of time!
So, they don't want to waste time with bots or 'people' who don't read their (often canned) responses. I'm sure your representative is concerned only with your well being and simply wants to serve you better, donations accepted with graciousness, of course...
This is exactly what
tacit described (and has repeated) many years ago in describing the misuse of html in emails. The logging of people opening them via tiny, transparent (invisible) images. But most seem not to care and leave html enabled all the time. I'm too paranoid for that and stuff still gets by SpamSieve occasionally! It is usually obvious when a message has an image that the sender feels is absolutely imperative to the future of mankind. When that happens, I simply verify who sent the message and then decide whether or not to view the image(s).
In this case, the politician may be completely unaware that this image was even inserted in his correspondence. It may just be a 'service' that is provided by the ISP/Emailing Service he uses for the statistical benefits it provides.
I doubt that asking him/her about it will shed any light on the matter. Most representatives have a hard enough time understanding the legislation before them, much less the intricacies of the interweb!
I dare say that you are much more aware and have a greater understanding of the interweb than the representative has about the subject of your messages to him/her.
