Probably a form of "read receipt" verification coming from the sender: Has Your Email Been Read? Read Receipts and Web Bugs
If you need to access internet assets (images, etc) in order to view an email, these assets can be personalised for your instance of the sender's email (e.g.
[host]/img/message[uuid].png). If you access the image, the sender knows that you (or your software at least) has read the email.
Privacy controls and filter lists within most email clients can block automatic loading of internet assets in an email. The resulting emails might look ugly but at least your privacy hasn't breached.
Note: The fact that the URL in question doesn't seem unique isn't an issue if this is being done by Google on the behalf of the sender (or themselves); as to fetch the HTTPS asset might require your login cookie, which thus links the image request to you.