How does the Sage E-Marketing feature determine that a mail has been opened?
Each of the email templates that you create or choose are stored on the Swiftpage server. On each of these templates, Swiftpage embeds a tiny 1x1 pixel image that’s stored on their server.
Each time a person opens an email this image is accessed on the Swiftpage server, so Swiftpage can keep count of the number of times the image has been downloaded from each different location.
There is more info here: http://server.iad.liveperson.net/hc/s-5126770/cmd/kbresource/kb-6845578160379664906/view_question!PAGETYPE?sq=email%2bopened&sf=101113&sg=0&st=646740&documentid=336329&action=view
The Swiftpage support website is a good resource for these types of questions: http://www.swiftpageconnect.com/Support/swiftpage-connect.htm