What is a user-agent?
Among your tracked data you will see a user-agent. But what exactly is this piece of data? Mozilla gives us the following definition: The User-Agent request header contains a characteristic string that allows the network protocol peers to identify the application type, operating system, software vendor or software version of the requesting software user agent. So it is some data that the tracking pixel also retrieves that tells us something about the device and software of the recipient of the pixel.
From a user-agent you can typically determine what device, browser and operating system is used. In the case that an e-mail service uses a proxy, a different kind of user-agent will be tracked, for example the GoogleImageProxy user-agent.