Some users may find that when a message is opened, they aren't being notified that it was read by the end user.
Nylas detects that an email was read by embedding an image pixel in the HTML of the email message. That image needs to be downloaded in order for us to detect that the message was opened. For most email clients, this is not a problem because they almost always accept HTML emails and will also download images when the email is opened. For the minority of clients, or for clients where the user has set specific preferences, we will not detect the open if A) the client reads text only based email and doesn't accept HTML, or B) the client does not allow images to be downloaded (sometimes for security reasons).
With Google accounts specifically, there is another reason why the tracking pixel may not be downloaded. Google decided to protect users from malware and to optimize image attachments for email by proxying those images and using a cache mechanism to serve a link to the same image.
Google decided to use this method of image expressing to save users and their server's bandwidth, avoid duplicates and optimize the client-to-server-to-client image processing. Basically, it stores a copy of an image, and then if some other gmail account requests the same image, it is taken from image proxy instead of the original location. Since Nylas can only detect a download from the original location, the open tracking doesn't work when Google serves the cached version of the image from their image proxy server.
Additionally, Google proxies the image shortly after receiving the email, so the initial open notification will be as a result of this action.
A bit of details on this can be read on Google's official blog: https://gmail.googleblog.com/2013/12/images-now-showing.html
Here is information about Google's image proxying from an external URL: https://litmus.com/blog/gmail-adds-image-caching-what-you-need-to-know