I too also had this issue and based on the solution provided previously.
Create the following typology rule to automatically add the pixel just
before the body tag to all email using the rule. if
(delivery.targets.filteringSchema == "nms:recipient") {var html =
delivery.content.html.source.toLowerCase().indexOf("");var
openFormula = getOption('NmsTracking_OpenFormula');openFormula =
openFormula.replace("$(urlId)","1");var pixel = openFormula;var newHTML
= insert(html, pixel,
delivery.content.htm...