I have a service, let's call it Trading Cool Stuff, that sends me emails on a daily basis. I set up an autoforward rule to send these emails to someone else, and that person does some NLP on those emails after reading them through gmail's API.
Can Trading Cool Stuff detect that these emails are autoforwarded?
The short answer is no, Gmail and most of the email services only track the emails until they reach the receiver's inbox. However, there is the possibility they are using some third-party software attached to the emails to track them.
Google Analytics offers the option to know when an email has been opened, but tracking the forwarding is not possible, at least from the Gmail API side.