Transactional Email adds a small, 1px by 1px graphic at the bottom of your HTML emails. When a recipient downloads images in your email, Transactional Email will record this as an open.
If you have enabled click-tracking for your emails, Transactional Email can also record an open when a recipient clicks on a Transactional Email-tracked link. Since it is impossible to embed the tracking image in text-only emails, recipient clicks are the only way to track opens for text emails.
Enable open tracking
You can enable open tracking using the track_opens parameter (API), X-MC-Track (SMTP), or globally for your account on the Sending Options page.
Global settings apply for all emails by default, but if the API call or SMTP headers have different tracking options, this will override the account setting.