Ask MailChimp: How do you track email opens?

Q: How do you actually measure email opens? Also how are you able to individualise these opens? Also obviously if people don't open their images this won't be tracked.

A: Opens are measured in HTML email with a tracking .gif. It's tiny, transparent, and virtually invisible to the recipient. When the image file is downloaded from our server, we measure that as an open. MailChimp generates a unique URL for each recipient, and uses that in the .gif file's  <img> tag for every single recipient. If people don't turn on images, or if they only view the plain-text version, the .gif obviously won't work. However, MailChimp can still track opens (even in plain-text emails) if someone ever clicks a link in their email (so long as click-tracking is enabled by the MailChimp user).

Plugs:
More information on tracking and email campaign reports in this free PDF guide. You might also be interested in MailChimp's enhanced campaign reports here.