I use OEClassic email client software on Windows, and by default, it will not download any images. I have to click a link on top of each email if I want it to download and show all the images. In that case, most of the emails will not show anything anyway, because for some reason the https protocol is disabled in my email software. So, if the image url starts with https rather than http, it will never be downloaded. I am not sure how many people use this same software, but I just want to let you know that using a 1x1 pixel image is not a very reliable way to tell if the email was seen.

If I wanted to know for sure if someone read my email, then I would write several paragraphs but I would only include part of the first sentence in the email to make the reader curious. Then I would include a link to click where they can read the REST of the email. If somebody clicks the link, then you can serve the page using a perl script, and you would automatically get an email-read notification that way! ;)