Mandrill adds a small, 1px by 1px graphic at the bottom of your HTML emails. When a recipient downloads images in your email, Mandrill will record this as an open.
If you've enabled click-tracking for your emails, Mandrill can also record an open when a recipient clicks on a Mandrill-tracked link. Since it's 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.