How does mailchimp count the number of emails opened?

Question

Email

Do they have Google Analytics in every email? Or do they send some kind of javascript with a letter that automatically pluses each time it is opened?

Answer 1

You can insert a link to the "picture" on the server into the body of the letter, and in that server there will be some script as the picture. It contains the counter incrementing logic. When the letter is opened, the mail client will try to upload the picture, and the script will calculate it.

Answer 2

Everything is easier. A single-pixel image is inserted into the letter. Link to a real picture, on the server, when accessing this file, you can see where the call came from, because each letter has a unique referal link to this picture. Thus, repetitions are excluded and unique hits are precisely counted.
There will be a miss only if the display of pictures is disabled in the mail client. Then this image will not load.