We track "opens" in your email by placing a tiny, transparent graphic at the bottom of your message. Whenever that graphic is downloaded from our server, we know that the email was "opened."
There are obvious drawbacks to this method. For one, it only works in HTML email (we obviously can't place any graphics in plain-text emails). Secondly, if a recipient had image-blocking enabled in his email application (a common privacy-protection measure found in most email programs nowadays), that means they didn't download the tracker image, and we wouldn't know it if he opened the email.
Is there any way around this?
Sort of. If you enable click-tracking in your emails (works in both plain-text and HTML emails), and someone clicks a link in your message, we'll register that as an "open." If you're relying on click tracking for opens, remember to set up your links properly. For HTML set up your links according to these directions and if you're typing out your plain text email by hand, make sure that the link includes http:// at the beginning. We're unable to track links in the format www.mailchimp.com. http://www.mailchimp.com is the way to go.