How does Litmus track their email analytics?

Question

So, \'Litmus\', a web app for testing emails and webpages across browsers and email clients, has a proprietary method that they claim is able to track not just opens, clicks

Answer 1

They know when the email is opened (it's when the image is called from their http server).

They also know what the user do and when since they can easily replace all links with their own tracking URLs redirecting to the original link.

There is nothing exceptional here. They are just a bit more advanced than their compatitors. There is no magic.

I have only one doubt: how they track delete. Technically, there is no way to know what happened to the message after it was read.

I suspect that a "deleted" mail is a mail that is never opened.