Open tracking works only for HTML emails. You can send a multi-part email (containing both plain text and HTML versions), but an open will only be tracked if the HTML version was opened. For every email you tell Postmark to track opens for, an invisible 1x1 pixel image is added. When the recipient opens the email and the image is loaded, Postmark will know that the email was opened.
Most email clients will download images by default, but some will not. If a person reads the email without images being loaded, an open even will not be tracked.
Postmark will record each time the email is opened, including multiple opens by the same recipient. Both unique and total opens will be displayed so you can gather both stats. Postmark will also provide you with the following data (when available):
- Geographic information
- Operating system information (Mac, Win, etc)
- Browser information
- Device and platform used to read the email
To enable open tracking, refer to the documentation.