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:
- the client reads only text-based email and doesn't accept HTML
- 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 or may show strange open results. Google opens and proxies images transmitted to their users and uses a cache mechanism to serve a link to the same image. Because Google proxies the image shortly after receiving the email, the initial open notification will often be as a result of this action.
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 the copy instead of the original location.
Since Nylas can only detect a download from the original location, the open tracking doesn't work or shows strange results when Google first retrieves the image (an "open" that is not made by an end user) and then serves the cached version of the image from their image proxy server (end-user opens that may not be registered).
More details on this can be found on Google's official blog and this post about Google's image proxying from an external URL.
Updated about a month ago