Need help for reimplementation of the Mailspring API

I’m currently working on a free reimplementation of the Mailspring API (especially because I want self hosted and unlimited email tracking). I have already figured out how the account system works and how the tracking image is added to the email, but I just don’t understand how Mailspring gets notified by the server when somebody has opened the mail. Can anyone explain how Mailspring gets this data from the server?

Update: I have now figured it out myself since it seems like there isn’t any activity here. If anyone wants to check it out the project can be found here