Note: Open tracking is only available in transactional message streams.
When logged into Postmark select the server, click on the message stream, and go to the “Settings” tab. There you can enable open tracking for that message stream.
Once enabled, all outbound HTML emails being sent through this message stream will track all opens.
You can modify the
TrackOpens field for a specific server's message stream using the Server API to edit an existing server.